Buy a metronome and a recording device. If you are serious about writing guitar songs, consider buying a Boss RC 2 Loop Station. A valuable aid in composing for the guitar, the Loop Station costs around $180, and it allows you to record and store 11 different tracks. Each track can be multi-layered with various guitar parts. The Loop Station's drum feature allows you to adjust the drum tempo. If the Loop Station is beyond your budget, an inexpensive metronome and a simple cassette recorder will do the job as well. The ability to experiment with different tempos and record your ideas is extremely useful.
Think about the guitar sound you are searching for. Original songs result from a combination of influences and originality. A Chet Atkins-style guitar instrumental will sound radically different from a heavy rock instrumental. However, one approach is to combine different elements and create your own sound. Experiment with different guitar tones and effects. A slow tempo guitar song works with a variety of guitar tones, all the way from clean sound to a dirty distorted sound.
Set the Loop Station or metronome at a slow tempo, and begin experimenting with musical ideas. Songwriting is a combination of hard work and inspiration. Guitar songs are usually built around recognizable riffs or identifiable melodies. One way to find the right riff is to simply begin experimenting with different riffs until you stumble across the right one. Don't be afraid to borrow some ideas from your guitar heroes. A different way to find the right guitar melody is to write out the chord changes first. Try different chord changes, which will help you create different melodic patterns that are more complicated than simple guitar riffs.
Write out an arrangement for the song. Once the guitar melody or riff has been written, figure out what the musical accompaniment is. One way to do this is to sketch out the ideas on your own. Again, the Loop Station is a valuable aid. It will allow you to develop the basic drum pattern. Experiment with different guitar and harmony parts. Think about the instrumentation for the song such as keyboards, horns and other guitars.
Find the right musicians. Once an arrangement for the song has been worked out, assemble the musicians that you need. Show everyone his part, and listen to his ideas and input. Practice the song until it sounds good. Hit the record button. If you are satisfied with the results, book time in a professional recording studio.