If you have any drawing experiance what so ever, getting a tablet for your computer would be a good start.. I use a combination of OpenCanvas (a japanese drawing/painting emulator software) and PhotoShop to add texture to my already painted materials.
There are a few ways to make textures.. two types I know of is Painting it all, and then there is putting things together using pictures.. the secound is easiest if you dont know much about painting.. though the first migth end up more styleish.
Anyway, there is little you can do on a 3D model with paint.. its good enough for pixelart even though its a bit slow to work with as for tools etc. but for textures you will at least need something as PSP. Gimp is fine, but there is no support I know of for tablets.. But works fine for the picture method. I would strongly recomend to buy Photoshop, if you've got that kind of money.
your HUD looks good though

Keep it up