Welcome to the Forum! You're asking a fairly broad question there as there are a lot of different kinds of patches and each has their own method. I'd suggest checking out the Tutorials section here in the Forum and in our Wiki for more information on patching and step-by-step instructions. The reason we've got them is so we don't have to type up that information again and again.
My advice is to pick one thing first, and learn that. Maybe making some logos in photoshop and progress onto other graphics. A lot of patching work is just research, it makes a patch much more realistic if you've got good info.