Foremost, you want to play at an poker site online that is reputable. If you are only playing for free, this might not seem as important if you are playing for money, but some poker sites that are not reputable may have you download their software that contains spyware and malware. If you are planning to play at an poker site online with real money, check to see if any of the professional pokers players endorse this site. People like Doyle Brunson, Phil Gordon, and Daniel Negreanu all endorse different web sites. If there is a professional who puts his name behind the poker site online, you can bet it is on the up and up. If not, it would ruin their reputation!
Other things you might want to look for in poker site onlines are those that let you chat with other players. You can learn a lot just by chatting or even just by reading the chat of other players. Some poker site onlines even offer you the ability to turn chat off, in case players get offensive.
Finally, you will want to find an poker site online that has easy to use software. If you try one site, but find the software cumbersome to use, check out another site. Eventually, you will find an poker site online that is perfect for you!