Database theory is a large, and sometimes complicated, subject. What part, exactly, are you having trouble with?
There are tons of webpages that deal with database design - have you tried googling?
 
Sorry, can't help with an open request...
 
AndyF

I agree with the above, - good starting point is a Google search with "relational database" as the search query.
 
Followed by- maybe tinkering with something nasty like MS Access if you have it. Or maybe MySQL (which is less nasty and quite nice really).
 
You can research SQL (Structered Query Language).
 
You can always download Oracle 10g and have lots of fun playing with it (see oracle.com)
 
Or we could all be wrong and you really meant "I need database therapy" which is something I say quite often lol