Here are a few general guidelines.

Betting limits
You need to find a table you can afford because you’ll be posting plenty of blinds. If your bankroll is $50, then you wouldn’t really want to play at a $5-$10 table.

Limit vs No-Limit Tables
It is probably best to only play no-limit tables if you are a very strong player with a large bankroll.

The players
It's a good idea before you start to play to watch the action, and learn about the players. See if you can spot the loose players, the tight hit and run players, the crazy players.

Pick your seat
Try and get a favorable seat based on the players at the table. You’ll probably want the unpredictable, aggressive players on your right, so that you can get out of a marginal hand before it becomes too expensive. It's good to have the timid players on your left playing after you, as you will be able to bluff and they will fold. If not that will tell you they have a good hand.

Know the odds
It's not too difficult to learn the odds for when you need one card to make a full house or a straight. Use this as a guide for whether or not you should stay in a hand.

Have plenty of time available
Don’t rush in for a quick half-hour session. You should study the players before joining the table. Give yourself at least an hour.

Don't play with scared money
Only play with money you are prepared to lose. You need to play sensibly. If you are afraid of losing money you will easily be bluffed out of good hands or play too timidly.

Be disciplined
If you are down don't chase your losses. If you are up don't play loosely because "you're playing with their money."