For a Shaman, you've probably leveled up with a mix of Enhancement and Elemental, or one of them - you will be needing to upgrade your gear with dungeon sets and heroic / pvp battleground items, however, keep in mind that Enhancement just doesn't work too well within the Arena, and no top teams ever do well with one in there.
If you're fresh just getting to 70 and want to start some Arena games, you would be probably be better off starting as a restoration shaman. Their healing abilities allow for decent competition in 2v2 and 3v3s, although if you have a good 5v5 lined up, you could also go for burst damage by going elemental.
Make sure to go for the heroic trinket, Icon of the Silver Crescent, as you move on up in your games, it's always good to have a damage and healing boost should the situation require it, and that trinket is the best right now for that.
Naturally, if you've chosen to go Elemental, pick out your gear with damage and healing adequately and run a lot more instances in your free time so you catch your damage up fast with your tiered ranking. This way, you will provide valuable burst damage that can win you the battle.
Popular Talent Builds
The easiest way, as mentioned before, to go into the Arena is being a Resto Shaman - good healing abilities and survivability give the resto shaman a good spot in groups. Earth Shield and big heals are always appreciated. Taking a resto shaman up with good gear and enough time spent in the Arena will make you very, very hard to kill, moreso than a Priest and almost on par with a well-geared Paladin.
The other popular option for a Shaman is Elemental. You will be needing a lot of damage gear without sacrificing your defensive potential. Your main role will be focusing down one target and killing it as quickly as you can, preferably with a big crit or a proc to finish them before heals or support come their way. Healing others is secondary, you will be essentially a mage, arena wise.
If possible, always try to have Nature's Swiftness for the Arena, it's a highly valued talent that lets you throw out a quick heal for extra support when damage is spiking too fast.
Make sure your partners have Earth Shield and all their appropiate buffs before you enter the match - it's also a good idea to request other buffs like underwater breathing and invisibility to be put ontop of yours, so if they get dispelled or purged, those go first.
Regardless of your spec, Ghost Wolf will make you hard to see and hence hard on the other team to figure out what kind of team yours is - you can take advantage and have your partner attack, or simply sneak up and attack yourself when they're distracted!
When facing classes that have fear abilities, don't forget to use tremor totems. Grounding totems are a must, but tremors will be nice to break out of many CC's early on without the use of trinkets.