As far as creating a layout, start with blank pages. Ask yourself "What information needs to be on this page?" Think what NEEDS to be there, then think of how you want to interact with it in that setting. This will help you decide what kind of controls need to be present, which can lead to a (rudimentary) layout that you can finetune once it's in place.
As far as colorscheme and detailed layout, pull up an existing project you've worked on and check out the UI. Play with the details, and try out things. If you see something and think "I wonder how this would look over there," move it over there and try it out. Change some colors to see how it looks. This will give you a feel for what works, what works well, and what just isn't a good idea.
As far as generating graphics, there is some natural talent and skill involved in that. Basic image-making isn't to hard to pick up, but if you want quality graphics, you'd better have some artistic skill (or someone available who does).
All in all, its about taking what you need a view/page/screen to do, putting the tools you need to do it in a box, and arranging that box so they all fit nicely. I suggest a lot of test use, to see if it is something you find easy and natural, or if you have to think too much. Make changes if you think it's best, or even if you just don't like what you have and don't know how to make it better, you might like what you see.