welcome to GD.SE!
To be honest, I don't think you really need to be a Photoshop guru to design your UIs. Especially if you are doing web, and you will end up coding everything anyway, the skills you will need to present some mockups and wireframes shouldn't take long to learn. Photoshop is quite intuitive, but this of course doesn't mean it won't have a certain learning curve. The thing is, if you are designing for web, you should probably do it in the browser and just use Photoshop for some graphics and for trying things in the layout.
You could start with some basic tutorials, just to understand how the program works (layers, text effects, shapes, things like that). Once you get it, you can follow the tutorials that guide you through a process and help you achieve a result, as they usually cover some of the techniques you will be using later on. Some tuts like these, focused one web, could be very useful.
Now about the concept, I always do mine in pencil and paper. Photoshop is great for certain things, but not for everything. There are much better tools for prototyping, such as Balsamiq (premium) or Pencil (free and open-source). They are faster and easier to use.