If you're completely new to boxing, i'd just suggest to try what you think seems most fun. It works well to get to know how it all works. Once you've figured that out and possibly found out if you like the class or not you can stick with them or make a new team.

Few questions to ask yourself:
1. PVE or PVP? or possibly both
2. solo heroics?

Depending on those you know what to pick.
for pvp 4 shaman, DK + 4 rets or boomkins is effective.
For pve 1 protadin + 4 ele's is simple and effective.

But things like 1 prot, 3 dps casters + 1 healer will work just as good if you configure it all well.