Welcome to the power of the "sub"!
My advice: take it slow. Convert a very small section of your site. Get comfortable with the process of converting and the kind of subroutines that you will need. See if you notice some patterns in how you go about converting things. Apply what you learn to the next batch of pages. You will surely learn as you go... and there are many here to help you.
Now would also be a very good time to learn about version control. In any case do lots of back-ups. Mistakes are a natural part of learning and you don't want to break your site while you are trying to make it easier to manage and expand.
Best of luck, beth