Why a sketchbook?
Thumbnail sketches are a technique I picked up when majoring in Graphic Design in college. Thumbnail sketches are a quick and easy way to try out a lot of ideas. You are not investing a lot of time or artistic technique in each sketch so you are not so committed to an early idea.
A pen or pencil is all you need to get started on your sketches
Just start. Even if you have ideas you do not like, it can be helpful to sketch them out so you can move on to better ideas. Once you have tried out a number of ideas as simple sketches, you can pull out some colored pencils, pens, or crayons and start brainstorming various color options.