A shaman's versatility can greatly aid a group in the PvP setting. Save your teammate with a quick heal. Do a massive windfury critical hit that your opponent is not expecting. Or, stand back and throw lightning bolts like Zeus himself. The opposite side of the coin is that you are expected to use all those abilities to help the group out. You should be throwing heals when you can, or when it is an emergency. You should be countering spell casters with earth shock. Remember, shamans are true jacks of all trades. Even an enhancement shaman can do some amount of healing.
Healing and Support
Shamans are in a group for support. They many be the main person dealing damage with spells, but you are still expected to help out with heals when you get a chance, or with emergency heals. You are also expected to use your totems when needed. Watch your teammates. If you are not the main healer, I would talk to the main healer when he starts to heal team members. This way, if someone gets too low on health, you know to throw a lesser healing wave. Also, talk to your group about what totems they have used in the past, and what totems you think should be used. Learn to compromise sometimes. Explain to your teammates why you use certain totems and ask them to explain their reasons to you.
Using Your Totems

In PvP you really need to feel out your opponents. Shamans can have a hard time if all they do is drop totems that get attacked right away and killed. Talk to your group about totems. They will have preferences, and you need to use your totem to your

group's best interest. Totems that buff the party are only good to party members who are within 20 yards of it. This can be extended to 30 yards with a talent point.
Gear
PvP gear is different from most PvE gear. Resilience and stamina is where the big difference arises. In most PvP settings, you will not need to worry about regenerating your mana. It will be more important to have a large mana pool you can use. Also, most PvP gear has a large amount of stamina. You trade stamina for intelligence and spirit on PvP gear. I recommend having two sets of gear: one for when you are out PvE killing, and one for when you are looking for a PvP fight. You should also carry a one handed weapon, a shield, and a two handed weapon. Enahnancement shamans should carry one more one handed weapon for dual-wielding purposes.
Battleground Tactics
The following are some tips and information for the battlegrounds that you might want to consider doing in the game. This is a good way for you to get PvP experience if you are not on a PvP server.
Warsong Gulch
Warsong Gulch is the first battleground that you can enter. It has the following brackets:
10-19
20-29
30-39
40-49
50-59
60-69
70
Your best bet is to always be at the top of the bracket before going in. It will make the battle that much easier for you. The battle is at least 5 vs 5 with the ablity to have up to 10 people on a side.
The goal of Warsong Gulch is to get your opponents flag back to your base where your flag is at. You can only score points if your flag is back in your base also. Shamans excel at low level Warsong because of ghost wolf. You can shift into ghostwolf after you get the flag and still be able to carry it. This helps out because no one else has a mount yet. Rogues still get sprint, and hunters have cheetah, but shamans are still sitting pretty good with ghost wolf.

Arathi Basin
Arathi Basin is the second battleground you can enter. It has the following brackets:
20-29
30-39
40-49
50-59
60-69
70
Arathi Basin is a 15 man battle over five resource nodes. Once a node is captured, it begins to tick resources into your side. The side that reaches 2000 resources first wins. The trick to Arathi Basin is to grab as many nodes as you can in the beginning, and then work on slowly pushing against your opponents nodes hoping to over take them. Arathi Basin needs to be coordinated. If one of your nodes gets captured, or is under attack, talk about who is going to help out so your whole team doesn't show up and leave your other nodes undefended. Your goal is always to maintain 3 nodes. If you can push your opponent so that you get all 5, the game will be over quickly.
Alterac Valley
Alterac Valley is the third battleground you can enter. It has the following brackets:
51-60
61-70
Alterac Valley is a massive 40 versus 40 PvP event. The battle is won when the opposing general is killed. Alterac Valley can take 20 minutes, or it can take two hours or more. Alterac Valley has many quests you can do to strengthen your side's NPCs. Most of the time, these quests are not even touched. It is normally a simple rush game. To maximize your honor and tickets from Alterac Valley, a rush is the best. It will take about 15 players to kill the general on their own. If all 40 players rush the opponent and kill as they go, it will be a shorter battle. This is an epic battle though. Don't expect it to be a walk in the park to the target general. Also, the way the board is set up, the Hordes do have a disadvantage.

Eye of the Storm
Eye of the Storm is the highest level battleground. It has the following brackets:
61-69
70
Eye of the Storm is a 15 person event. It is a combination of Arathi basin and Warsong Gulch. There are four towers that need to be captured to gain resources. Then there is a flag in the middle that can be returned to a tower occupied by your side for 75 additional points. The first team to 2000 points is the winner. The strategy for Eye of the Storm is about getting towers, and then defending them while you send a small group of yours to grab the flag and run it back to your tower. This battleground, just like all the others, is about communicating with your group. Know where you are needed and what needs to be done.
Arena
Arena fighting is last man standing. You go in with your team, they come in with theirs. The match lasts until only one team is left alive. All cool downs that are longer than 15 minutes are not usable. Arena fights are not always against opposing faction members. You can end up fighting a team from your same faction. All consumables that are not summoned cannot be used. After the battle, the team will receive an arena point adjustment. Every Tuesday at midnight, all members of a team that were in at least 10 matches, and members who where active in 30% of the matches receive arena points depending on the team rating.
Teams can consist of 2 vs 2, 3 vs 3, or 5 vs 5 teams. You can be in one team of each type. You must be level 70 to be on a team, and to receive arena points. Anyone of any level can play in a skirmish to just test themselves. Also, all teams can have twice the number of players required on it. So you can have four players on a 2 vs 2 team, six on a 3 vs 3 and 10 on a 5 vs 5 team.
There are also 3 different arena settings. The Ring of Trials in Nagrand, Circle of Blood in Blade's Edge Mountians, and Ruins of Lordaeron in Tirisfal Glades.
2 vs 2
A shaman is probably going to want a warrior partner, or another shaman. Warlocks are also a good choice, but they do not complement each other like the other classes do. Elemental shamans might work with a mage to do long distance attack together. Enhancement shamans might work out well with a paladin, or a priest.

As a shaman, you are going to want to be using your grounding totem and helping your partner out. Coordinate attacks, and remove either the healer, or the high DPS member of the other group. A shaman is commonly going to be the focus of the other team. They will want to remove you first due to your versatility. Make life hard on them, and just switch into survival mode. Keep healing yourself, and hope your partner is taking care of them. Remember to use tremor totems to break fear effects.
3 vs 3
Shamans are going to want to maintain the same classes they had in a 2 vs 2 team, but they can add one more. Go with a class that adds to your team. If you are enhancement and you have a warrior with you, get a priest, or a paladin. If you are elemental and have a mage, get a warrior who can draw the fire at first. This gives your team a chance to take out one of their guys before they realize you got two heavy hitting casters. Even 3 man shaman teams can work out. Have at least one restoration shaman, and one enhancement shaman, and the third can be whatever. Two restoration shamans with one enhancement shaman can be a strong combo with the totems being all over sucking up spells and healing everyone. Two earth shields being active on the group, and the ability to cast earth shock and lock down a spell caster can be crazy. Also, think about laying 3 tremor totems in different spots because this means more likely it would break a fear. This is a strong combo that is seldom come across.
5 vs 5
This is where you are going to want to start looking for a balanced group. Just like when you are doing high end instances, you want a group that is well balanced and can deal with any situation. Crowd control is very important in 5 vs 5 battles. If a team can control 2 or 3 of their opponents while their team has gone without any crowd control it will be a much faster fight. Because of the importance of crowd control, you will seldom see more than one shaman on a team. Shamans can kite people, but have no way of removing them from the fight. 5 vs 5 teams provide the highest amount of arena points at any given ranking. If you can get a solid group together to do a 5 vs 5 arena team, you are well on your way to getting your arena gear.