for me it's kinda like for Amanda...I get some melodies stuck in my head for weeks then I go to the piano and play them.
Or I just slip my fingers trough some interesting notes, and then as soon as my ear picks that up , I try making a full melody, I won't deny that having musical theory knowledge helps A LOT, because after u absorb the theory , the rest flows by intuition.just play with the piano, don't be scared of hitting wrong notes , cuz that will make you find interesting things out on your own
Try choosing one scale and then it's relative minor, for instance, and pick 5 or 6 chords from them and make a song, that's a good exercise for starting.But if you are completly clueless , I think picking up a book about harmony,scales and chords and all that will help you a lot =)