Where to start.

Pick a key.  Any key.  How about E-minor.  How about A-minor?  B-major's always good.  Throw in a 7th, you can't lose.  Now your going.

Now pick a 4th or a 5th up or down.  Drop by one.  Go to the 7th, riff.  There you go, you're writing a song.

Repeat and have fun.