Definitely get a MIDI keyboard, I just ordered one myself. It is very hard otherwise to create a nice melody without one unless you're up for a lot of extra work or are really good with playing on your computer keyboard (not recommended).
I suggest getting the Korg Nanokey2 Slim-Line. It's a basic MIDI keyboard, 25 keys, and few extra controls. It's only $50, I just ordered one myself.