I'm pretty new to but,I'll atleast try to help.
There are a rather large number of units,as to how to use them and which to take is down to the combos/synergies you would like to create.Perhaps you could check out BattleCollege,they have rather nice articles explaining basically how X unit works and it's uses:
http://battlecollege.wikispaces.com/
Also if you are still undecided on which race to get,perhaps ask the local
PP guys to do a demo of a race for you.I'm sure they won't mind.
As for starting it,I would advise that you not make any large invest until January.This doesn't mean don't buy things,it just means don't buy duplicates of the same unit.The best place to start would most likely be a battlegroup/warpack for your selected faction.
I hope I didn't ramble too much and that it helped a bit.