you can use Massive for anything, its an awsome powerful synth, used right. tbh as you are starting out it doesnt really matter to much what you use, Massive will do you for both melodies and basslines, pads, whatever you want to throw at it. load it up, plug in your keyboard, and play a few notes, takes time and no one can really tell you how to write a good melody imo, but try chords to start, Major (eg C, E, G) and Minor (eg A, C, E) and play around, have fun!