Hey I just made my first theme song. I made it very short but may try to make it longer. This might be used for my next game.
http://joelphilippage.googlepages.com/Music1.wav
Well it's a good start, but maybe you should add some other instruments? I would say that a guitar would be nice in the background for example. Also it would get very repetive if you just hade the same melody all the time, so try to change the melody after you've been playing this line for maybe two or more times.
If I where to do this song for example, I would first let the melody you've created now play alone, then the second time add a guitar who played some chords, and then change the melody a bit, or maybe add some easy drums.
Also, I would shorten the note that you are holding out near the end.