What do I need to start customizing?
Lots of patience. It doesn’t happen at once. Start with some simple projects (repaints) and then take on more difficult chores. Also, I recommend you visit the sites of numerous customizers. Most will give a fairly detailed accounting of the techniques they use, and most are also happy to answer any specific questions you might have.
Easy: A simple parts swap. Little if any paint.
Basic: Some modified parts. Mostly a new paint job.
Moderate: New and modified parts.
Skilled: Lots of new and modified parts. “Frankensteining” often involved.
Advanced: All of the above, plus a great deal of original sculpting.