0)goto modeling set(assume you are there if you already have a mesh)
1)first make a UV map of your model (you can just use polygon UV's-> automatic mapping)
2)Switch to rendering set
3)Go to texturing->3d piant tool->option box
4)click assign textures
5)choose a resolution like 1024 * 1024
6)paint
when you save the scene a map has been automatically made. You can then just open it up in photoshop and paint more on it.
**Theres about 30 other ways to do this, but this is about the quickest way
I didnt know what the heck I was doing when I first got maya, untill I paid $500 for a day long seminar then I decided I could just learn things myselfby reading books.
Good luck if you have any questions about the process I explained then I will answer.