Start off with design concepts. Have an idea of what you want to make, maybe a sketch on paper or some pictures of characters you'd want to pony-fy or symbols to reproduce.
Here's some links to get you started!
FAQ Tip of the week # 8 - Making your first custom pony
My Little Pony Trading Post - FAQ: 5.0 - Restoring and customizing ~ How do I ... 
The TP FAQ on customs
Questions from a beginner customizer
For brushes, I have a variety. Michael's crafts sells a multi pack of 25 brushes for $5 that are a good starter kit for beginner customizers. Also you'll want to get yourself at least one sponge brush if you're attempting FBRs (full body repaints) and one teeny tiny brush for doing eyelashes.
I use Golden brand sealant, in matte. I also have gloss but I only use it on areas that I want to be shiny.
Cleaning and cancer advice here:
My Little Pony Trading Post - FAQ: 3.0 - MLP Care ~ How do I ...
Tutorials are up above, in the blue bar below our banner ^ *points skyward*
(I'm not called the FAQ pony for nuthin!) ^_~