I started on PartyPoker - plenty of fish on there, but I still lost most of my money playing HoldEm... nearly ran out before clearing the 100% bonus...
When I switched to Pacific I'd started playing Omaha High/Low (using strategies based on Play Poker Like The Pros advice), and started doing a LOT better.
Pacific's not a bad site to start out at for real money, since there are micro-limit cash games which aren't so dissimilar to the (abysmal) level of play on the Play Money tables...
I'd recommend trying some low-stakes SitNGos, $1/$2 tournaments, or micro-limit ring games - work your way up gradually - and remember to make sure you're playing at stakes your
bankroll can afford.
All of my friends who play online for money started out on the play money tables, and won at those, then switched to real money and lost money on average over their early sessions. Those that are willing to play to some kind of strategy are now in profit, but I'm saying it to impress the idea that most so it's not all bad, you just need to play better than on the Play Money tables. On the plus side, bluffs almost never work on Play Money tables, but a well-timed one (against the right opponent) becomes a nice addition to your poker arsenal once you make the switch to real money - personally I try to avoid the temptation to play on play money tables as they're not as much fun as the psychological warfare that takes place on good cash tables.