many books have been written on the basics of planning a quilt (including designing etc..) have you check your closest library... I started with a book from Harriet Hargrave and Sharon Craig and I know some other people will recommand other books.
It can be a long process but a very nice learning one. My avatar is my first quilt and it is a design that just grew with blocks and design that I wanted to learn.
Go for it... just start and you'll learn and see what happens.
Even on this board, I am sure somebody will show you some explanation that have already been posting and that I am not aware of.
do you know about this site (and there are many others also)
http://quilting.about.com/od/beginni..._Beginners.htm