Unless your current site is in an MVC framework already, you basically have to start from scratch. But, the good news is that you've already got your HTML/CSS which is half the battle.
Here's my proposed steps:
1) Sit down and really understand the differences between models, views, and controllers -- i.e. understanding what each is used for and how to determine what code goes in which section 2) Rethink your current site in an MVC-mindset (don't try to do this while in step 1) 3) Code your models 4) Code your controllers 5) Code your views
