To expand on Calvin's point (which is a great method), standing in a room with your band and ask the drummer to "give you a beat." Then start jamming to it, and finding bits of stuff that works. Not lead, but riffs, parts, chords, etc. Keep playing it, fine-tuning it and having fun with it, and then when you get tired of it, see where your head/heart/hands go next. Allow yourself to change and see what happens. That's the easiest way to find parts that flow together in a song-like manner. Once you do that a few times, then you can stop and figure out your arrangement, and from there the other members can start fine-tuning their parts over or under it.
Then the singer comes in and changes the whole arrangement, but at least at that point, you have the subframe of the whole thing in place.
