Learn to use your ideas in musical notation instead of using tablature (written-out fingering). Tablature is fine when an entire ensemble is playing and the guitarist needs to improvise chords; however, tablature is not effective for creating rhythms needed for a theme.
Study chords specifically used on the guitar, such as the G6 chord that consists of a G, B, D, G, B and E. Learn about other guitar chords that appear commonly in guitar music, you will need these to set chords to your theme.
Write a theme by using stepwise motion and experimenting on the guitar to come up with a theme. Some good guidelines are to use mostly stepwise motion and avoid large skips. If you do use a large skip, don't continue the skip in the same direction, move in the opposite direction after the skip. This will help create a suitable contour.
Add chords to the theme by looking at the main beats of the melody. Use a chord that fits two characteristics: Have one note that corresponds to a melody note and at least one tone in common with the previous chord. Common tone progressions allow you to write logical chord progressions without much effort.
Create a memorable guitar riff to differentiate your song from other songs. A guitar riff is a short musical idea that is easily recognizable by its rhythm and melody. A riff may contain only two notes as long as it is memorable. Simple riffs usually end up being more memorable than complex ones. A good way to write a riff is to tap out a rhythm first, and once the rhythm is established, add pitch to the rhythm.