There's a few ways around this but it very much depends on having a clear view of your intentions.
Do you want a user to have a user area within the site that holds content particular to that user, or is your content going to be available to all users but you want the site to personalise it by using their name (after login) on pages and so on ?
If you can clarify what it is you want, a solution could be easier to recommend.
In brief, I use user sessions to personalise content. A user gives their username (for the sake of argument) and every page after that welcomes that user by name and displays information on where they've been within the site and so on.
Alternatively on a large site with many registered users it might be desirable to have a user enter the site and be redirected to their content (username/password combination matches redirect location and permission to access it).
Can you give more info ?