First you need to have the UV layout completed before you do textures. There are many different ways to create a texture once you have a UV layout. One way is to export the UV snapshot to Photoshop to use as a guide to paint. Another way is to use Quixel suite or Substance Painter, try looking at tutorials for one of…
You'll need to create and unwrap UVs for your model, there are many tutorials available, here's one that covers some basics and shows you how to split edges to get a good layout. http://www.youtube.com/watch?v=ya1LDh9cZOs