1. Start with a chord sequence. C G Am F is a good one to try, but experiment with the chords in my previous post to see which one you like.
2. Play these chords into Garageband and loop about 4-6 times.
3. Play them back to yourself and try humming or playing a tune over it that fits.
4. HINT: The tune should have a lot of the same notes as the notes in your chord that is playing at that time. e.g. If the chord is CEG, then the melody should have C E and G in it.
5. Try adding a baseline. This should be the bottom note of your chord so, if your chord is CEG, your baseline note should be C.
6. Develop your baseline note by playing it in with an interesting rhythm.
7. Add a beat. NOTE: you are NOT allowed any sample beats, make your own!!
Good luck. Also, for ideas on good beats check out the youtube video below.