In order for you to start writing your own original pieces you have to understand the structures of a song... and you have to know something about theory. Learn a bunch of chords and a few scales... try to understand what scales to play in certain keys a song is in... then when you get around to learning the basics of that you can put some chords together, write in some lead fills with the scales you know, and come out with a finished piece that should sound decent for a beginner. As it stands now, I don't know if just being able to read tabs will give you what you need to write your own music. Don't think that I'm telling you to NOT try out your own stuff... if you've got good ideas try to bang them out. Just try to learn a little something about what structures a song in terms of theory.