Dude what kind of a question is that?? Do read this *www.thinkdigit.com/forum/viewtopic.php?t=25203 And please, treat RAM (in caps) with the due respect it deserves!
Jokes apart, you have to first see
1) the kind of a computer system you have (processor, motherboard)
2) the purpose of the upgrade (games/net/office etc)
3) budget (if you had the money all the RAM (hehe) in the world)
Then you actually go buy the RAM. But that's another story. Do post your comp config so that the members can guide you better.