Songwriting for Guitarists
First, I'd like to say that this is sort-of specific to rock guitarists, but could be applied elsewhere, I guess.
Ok, so you want to write a song? That's no simple task.
The hardest part of having a band is writing your own music (or possibly finding time to practice). There should be strong correlation between music and lyrics for a song that sounds good and expresses your ideas.
I always start with music. This way you're not trying to write music to magically match your lyrics. I'll write a melody, then depending on the mood of the music, I'll find a lyrical topic that fits the feeling of the song. This way I also know how many sylables I want per line.
Let's go back for a second to music writing. Start with a simple melody, something that you think sounds good. If you want to you can add riffs to your melody. Riffs in rock songs are often written in regular minor or major scales or, (most popularly) major/minor pentatonic scales. For example the predominant riff in Layla by Cream is written using a C Major scale. Once you have a nice verse progression, add variations as you see fit, but don't overcomplicate. Next, figure out progressions in the same key (most likely) for the chorus, bridge, prechorus, intro, outro, or whatever else you want in your song. I would suggest getting familiar with pentatonic, blues, and other scales for the purpose of soloing. I won't go through how to solo here, but I'm sure someone has written upon it.
Ok. Now you've got some music, a complete song minus vocals/lyrics (and of course drums and bass). Let's say this song is fast, kinda poppy, and upbeat overall. So what are some things you might want to write about that produce feelings similar to those expressed by your (currently) instrumental song? Write down a list of ideas, and get to work! I can't even begin to tell you how to write lyrics (try Demeter's write-up). That's not something I feel I can teach.
I'm a guitarist primarily, but I sing lead vocals. This is not something I feel I'm especially good at. I'm not saying I have a bad voice, but figuring out how to sing something, what music you will sing the vocals to is a daunting task. I find that trial and error, though time consuming, gives me the best results. It's just a matter of finding something you're satisfied by. Hope this was helpful, and good luck.