If you have the equipment to do it with...
Load up some drum loops into a audio application and cut and paste them so you get a 3 minute loop going. Divide it into sections that represent verses and chorus's.. ie, a slightly heavier beat for the chorus.
Set the drum loop going and start playing whatever comes to mind.
Try to think in terms of the way a song is constructed... a couple of bars of introduction, 4 bars for a verse, maybe 2 bars for a chorus..etc..
After a bunch of tries add some bass and keys to what you did until you start to build up a song.
After a few days it'll start forming itself together sort of like painting a canvas... it takes weeks sometimes.
While you're doing that, lyrics and melodies will start coming to you.
Try to just work on one song at a time and don't listen to other music in the meantime. Let your mind wander and play the song subconsciously while you're doing the dishes or whatever.
After about a month, you'll have about 50 tracks built up making up your song.
Now just go back and start shifting things around, taking tracks out and mixing it. Add your vocals and harmonies and voila, you've got a finished song.
Then just master it with t-Racks, convert it to MP3 and post it on the net.
Register the song with a performance rights association and offer licensing to various people like advertisers, film/tv people....
If you're lucky, the song will get picked up and you'll get about a years worth of pay in royalties cause your song was used on the newest teen tv show or whatever. Now you can ask for 100 grand in publishing on your next song, form a band and take private jets all over the world.