Writing songs is probably something a lot of garage bands aren't exactly into. And hey, why shouldn't they be? It invovles writing poetry. I can't stand the stuff.
But what's probably harder than writing non-corny poetry, is getting your own riff going. And what really s@cks is how when you do get something original going, you discover someone already did it. In fact, this happened to me once. I had a reverse arpeggio thing going, and I thought it was my own... but I thought I had heard it before.
I discovered two weeks later that it was in a Coldplay song - only the notes were ascending in the song, not descending. S@cks, doesn't it?
But, on to the songwriting:
Get started on your song, the bare bones. Do you know what lyrics you want? I'd suggest having some rough (at least) lyrics written out for your song. Whether they're yours, your drummers, your friends, whatever, you have to be honest with yourself: can you sing those (if you're the singer) in front of however many people at a club? Do you believe in those lyrics? You should. If not, the song will simply suck. And that principle goes for covers too - you have to believe in the lyrics.
Now, onto the tone:
What kind of tone do you want? Acoustic? Clean? Distortion, overdrive, bluesy overdrive? Death metal-ish? Figure it out.
As a starter, go to your amp, and set all the balance controls at five. Play a familiar riff, or maybe a small ditty of your own. Now think, is it too trebly to your song? Too bassy? Not enough midrange? Play with the controls. And I suggest you sit directly in the range of your amplifier. In my experiences, you will hear what the amp is putting out if you are directly in the sound range, instead of being farther away, where the sound can get all jumbled up from furniture.
When you find the right balance of controls, get working on distortion levels.
What kind of song is yours? Alternative? Punk? Metal, or Hard Rock? How do you want your song to spill out?
Think it out. Is it going to start with a nice, slow acoustic intro, or get right down to business with insane distortion? Once you've got this decided, draw a diagram - intro, chorus, verse, and name the tones behind them. That way, you'll keep everything straight, and will make it easier to modify in future.
What tuning you want to play in may not be a major decision. Tune down if you want heavier, drop D if you want to play some lower power chords. It just takes a bit of experimenting in riff mode:
Making a Riff:
Making a riff isn't so simple if you don't listen to music at all. If you like a certain bands sound or feel, and want to develop a sound from them (which everyone has done), then all you can do is learn to play some of their songs. You will be able to learn chords and riffs from them, how their guitarrist approaches the guitar, and spin your own twist to it.
Examine the chords the band uses. Power chords? Fourth chords? Sus2 chords, minor chords? Find out what ones you like. Don't be afraid to mix them together. For example, you could play some suspended chords and minor chords one after another, and see how you like it. Crunch out some power chords. It doesn't hurt to try and figure out what space you want between the chords - how many steps? In my experience, playing power chords on the 3rd, 4th and 5th string, then moving that down one string sounds nice, but that's my opinion. Your might differ, so experiment.
Remember, mix chords together, do whatever. It might also help to go to a "chord finder" site to find out different chord formations for the sound you want.
Solos should be the last thing that goes into your song. Only write a solo into the song if it adds to the song. Don't throw in a flashy solo because you want to play a solo, put a solo in because a solo belongs in the song. It should add to the feel, not make the song feel longer.
The basics for all solos should be the pentatonic minor scale. It may only belong to blues music, but Smells Like Teen Spirit by Nirvana was largely based on a pentatonic scale, with a power chord twist. The solo is almost entirely a pentatonic scale. So never give up on it.
I wouldn't suggest using a pentatonic scale exclusively, you have to be inventive to give it your own feel. Experiment with different positions, as each one has its own shape and feel. So, go nuts, experiment, and I would suggest hooking up your comp to record on your PC (most windows have "Sound Recorder", primitive but useful), and improvise. Then play it back.
Is there wasted space? Notes that shouldn't be there? Most improvised solos are consisted up of stuff that are made up, while thinking of an idea to do something neat. So play it back, with a pencil in hand, and if you pick up the good stuff, and ditch the garbage, then you should end up with a nice polished solo.
Remember, solos are meant to convey the point of the song in music form. If the listener doesn't know what the song is about by just the lyrics, the solo should come in and say, "You don't know what it means? This is what it means!" with attitude. Don't be afraid to let your music mean something.
Remember, music is how you express yourself, so as you look back on the polished copy of your song, you should be able to say, "This is me, and I want the world to know it!", unless you plan to be an underground indie group. But the principle remains the same.
It also wouldn't hurt to record small bits of it, and upload it to the internet/share it with friends. They should review honestly, and take their opinions seriously. If your song means the world to you, but nothing to everyone else, its not going to go anywhere. It'll be the pride of your life, but everyone else will think it s@cks.
Another thing is, don't take your song too seriously. Don't expect it to become another Stairway to Heaven. You might strike out, and you might hit a home run. You can always submit it to radio. If they don't like it, don't give up hope. It may yet still make you thousands in bars and clubs. Similarly, it may not do you any good in bars and clubs, but if you can send it off to radio, they might like it. It's all about the clientelle of radio listeners and bar-goers. So don't limit your search to one specific place.
So, take songwriting one step at a time. Be patient, don't give up, and most of all, have fun!