A. Use a programme such as photoshop or GIMP. Then use tuts to learn basic textures.
B. Dont really know, but fully custom models are very difficult.
C. Create the texture and use a programme like signs or Magic FPS. Its alot quicker and gives good results.
D. I dont really know.
I think your going ahead of yourself here, if your not experienced in 3D creation, you wont be able to create characters straight away. Start of with Boxes or easy to create objects with simple textures.
Use alot of tutorials and get used to the programmes before attempting anything major. But good luck.