Decide whether you want to play for free or use money. Obviously, if you decide you want to gamble you should always be prepared to lose, so don't put in more than you can afford.
Download anti-virus protection--this is a necessity.
Find sites. There are sites where you can play for free. Some are listed in the resource section.
Open an account if required. Most of the time it will be for a free account, but some sites require you to deposit money to play. These are good and bad sites, since you have the chance to win--or lose.
Download software, if necessary, to play on the site. Some sites require you to download their software even to play online, Make sure you computer has enough hard drive space.
Decide which game you want to play. This will also help you to choose which site you want to play at. There are numerous sites and you have to narrow it down somehow.
Play your game. Have fun especially if you are on a free site. If you have chosen to play for money, watch your money and try to quit when you are ahead or at least when you have broken even.