No simple answer. Trial and error, excellent record keeping, a couple of years.
You have to find yourself.
Pretty much what quads says - as you play more hands and more games, if you keep good records it should be apparent what your most profitable games are. It's different for everyone.
The lowest (ie: $1 SnGs) level tournaments at Full Tilt do have one quirk though: they charge 25% rake, which is higher than the bigger buy-in tournaments and probably their ring games as well (don't play them so don't know for sure). The high rake does make it harder to turn a profit at that level, so ring games might be a better option if that's the level you're looking at. Once you get to around $3 tournaments, it evens back out to the more normal / reasonable 10% rake.