Thats a bit too deep to go into for a forum post.

http://en.wikipedia.org/wiki/Database_normalization

Read that. Look at section 3 Normal Forms. Each of those has a link to an explanation of each one. Read the ones you're interested in. 3NF is probably the one you want.

Having read those if you have any questions then post back and I'm sure someone will be able to answer your question.