[King of Snake]

if you have a musical hearing you can get away with doing stuff by ear rather than by theory. Knowing a bit of theory never hurt anyone though.
Start by learning the theory behind scales and chords and playing them on the keyboard. Just by playing around with scales and chords you will get ideas for melodies.
Also, listen to records in the style you'd like to produce in and analyse how they work. What sounds do they use? Where are the sounds placed? (both in time and in the stereo field). How many sounds are playing at once? How do certain sounds work together? You could also try recreating bits of songs you like just to find out how they work.