Start with FREE Play until you understand the software
Real money players come to expect other players at the table know the basics of the game. Slowing the game down and/or asking questions because you haven't learned the basics, will likely get some nasty comments. Read the instructions first, play at a free table (all the online poker rooms we recommend have free play) to get the hang of things, then go play at a real money table. Other players (and your pile of chips) will thank you.

Watch your language
Most rooms censor chat so you can't use words like XXXX, XXXXXXXXXX, XXXX because they'll be XXX'd out. Remember playing online is the same as playing with friends, so it's polite to keep it clean.

Don't overdo it on chat
Chat features are for chat, but not everyone enjoys the distraction. Also a number of players use chat to harrase other players. So try to go with the flow of the table. If the crowd are is chatty, talk away. If people aren't, don't.

BTW - Don't be wasting time typing when it's your bet, it's rude and others will let you know it. Being social is nice, but there's a time to play and a time to talk.

Don't be a jerk
If you needed to be told that, you're probably not ready for poker. Don't talk trash to others, or 'zing' people when you're winning.

Try to keep pace
Most online poker rooms allow 30-60 seconds for a player to take their turn. This time limit is there event of internet delays or the occasional time when drift off. The time is not there to be used up on every turn - remember people are waiting for you, and games can get frustrating for all when people don't keep a good pace.

Most online poker rooms will allow you to select your upcoming action before it's even your turn, so do just that when appropriate.

Don't talk while a hand is in progress
Folding then talking about your hand, gives the remaining players unfair information. Saying 'and I had two aces!' tells people there are two fewer aces left in the deck. Speculating on what players left in the hand have also tips people off. Don't do it.