It's always a good idea to search both the forums and the internet before posting a question. You might find it's been asked before, and you'll have your answer straight away, without needing to wait for somebody to reply.
There are various threads in the Internet Marketing which deal with this and other keyword-related questions.
A quick internet search brought up these two results at the top of the list:
Once you've read these, if you still have any specific questions which remain unanswered, please feel free to post again. Meanwhile, I'm closing this thread.