If you are familiar with a lot of repertoire, you will have a treasure trove of starting material. Learning to analyze a piece of music is a great introduction.  This is one way you could be a way to start a new piece for the guitar. In this way, you have a map already laid out of where you are going, which chord you are going to use, and when and how you will modulate. By starting with an established piece even the form is given to you.