This advice is based ONLY on Limit holdem.
Before playing you need to understand your approach and bankroll strategy. How much you put on the table at any one time can be calculated by taking 5% of your total bankroll. For example, if you want to start with $200 you will be playing with $10 a table.
You are looking for a table where you can buy in for at least 50 big-blinds. So with our example $10 you will be looking for a $0.10 / $0.20 table. Sure its not glamerous, but work up slowly and as your bankroll increases then raise your limit using the 5% and 50BB rule.
This article is not about improving your game, there are plenty of books and forums out there on how to play better poker. We are assuming you know he basics and will play only premuim hands.
Table selection online is the most critical decision. Playing later in the evening will highlight this selection. You are looking for tables with at least 8 players and where at least 2 players have low-stacks. So look for players with only 5 or 10 Big blinds, so in your $10 example you want to see players with $1 or $2.
This is the key, these players will generally play loose and are close to busting out. They will play Ace-Rag or other specaltive hands, so wait your time and play hard against them when you have a premium hand.
Set your self a table goal. So if you buy in for $10, cash out at $15. Once the low-stacks have gone, then leave and find some other players in the same situation.
Keep track of all your sessions, either via a tool like PokerTracker, or write them down. You will find certain times are better, the late evening when people have been drinking and not playing well will be very profitable.
I have found just by selecting these low-stack tables late at night to be profitable, all the way to $2 / $4 limit.