I have used both intertops.com and bowmans.com, for mainly sports bets and some casino gambling.
Most online casinos have a "Free play" version of their games where they allow the users to get comfortable with the game, and then try it out with real cash.
You should check out,
http://www.wizardofodds.com, for strategies and facts about each online casino game you are interested in playing. They also have facts and tips about the real casinos.
If you do sign with an online casino, look for casinos that offers bonuses for first time clients and the amount of cash you are depositing.
Also, set a limit in terms of how much you are willing to lose out on and stick by it.