The most basic way is to probably just start with a simple chord progression. Then hum out a melody... or harmony if you really wish... And then fit some words into it. Before you know it you'll be able to tell which words fit where and what chords are best for your voice and so forth. Finding that stuff out is pretty important to finding your own individual style. Not everyone can sing multiple genres of music so just find your niche and get creative. Having some lyrical ideas jotted down helps sometimes too. Sometimes you can't fit something that you write down with the music you want... so you'll have to adapt it. A good exercise for that is to write a phrase like.... "My alarm clock woke me up at 5 this morning" and writing it in as many different ways as possible... like "5am, the day begins, my ears bleedin" or "the buzzin around in my head... make it stop 5am" just mess around with stuff like that and some simple chord progs... then work in some riffs and stuff. It helps too to get a cheap 4 track recorder or something to record ideas on....