I'd suggest SNGs purely because the strategy is easier. Ask in the tournament section and you'll certainly find people more aware of the strategy than I am, but to develop a winning strategy, provided you've got your head screwed on right and can figure out which sources to believe, it wont take you long.
Cash is great too, and you only need a really basic strategy to beat the micros which can be further simplified to a basic pre-flop strategy and just not spewing chips horribly post-flop. Still, that's much more time consuming to learn than SNGs.
The stakes you begin at don't matter much, it's more a matter of what you can afford. I recommend using very cautious bankroll management.
The best sites to play are probably full tilt and poker stars
, big sites are both reliable and give good bonuses (if you pick full tilt, make sure to sign up with a rakeback affiliate, see rakebackpros.com, which is the one most people use).