May I make a suggestion?
Instead of trying to build a CMS as your first project, its often best to start with a smaller project and increase the level of difficulty.
Why not try to start off building smaller applications first. For example:
Hello, World (just to get your feet wet)
Guestbook (good for form and database)
Voting Booth (form, database, administration module)
Appointment Calendar (date/time, database, form, administration module)
Jumping in with a CMS as your first project will likely be a frustrating experience. If you don't believe me, check out all the PHP CMS scripts and look at the sheer size of the task. Nine times out of ten, people bail out of these sorts of projects because they become so much work. They eventually give up and use something like PostNuk e, Drupal, Xoops, etc. instead of reinventing the wheel.