Hey Cowby,
I Agree with above guys ... its different for everyone ... generally for me I start with some melodic or chordal or rythmic idea that gets me inspired ... this could be anything ... then I start to add stuff to it ... and come up with a complete "PART" ... (There's a concept in classical music theory called "themes and variations" .... which is probably the closest to the method I use ... I do get kinda bogged down with it sometimes.)
Then I try to come up with other parts (the variations/or totally contrasting parts) that would fit to make a song out of it.
The main thing to remeber here is that usually a "song" or instrumental piece has distinct parts that are arranged in some order (and with some variation ... depending) ... like the "standard" (verse/verse/chorus/verse/break/verse/chorus type model ... the order isnt as important as long as there is some structure).
Some "concepts" (not necesarily in order of importance)
1) Stucture
2) Theme (The main Musical Idea)
3) Variation (Variation of the main Idea)
4) Contrast (A part of a song that contrasts or "sets off" the main theme)
5) Emotion
6) Movement
7) Style
8) Dynamics (Loudsness/softness)
9) Empty Space (In DJ music sometime there might not seem to be alot of it ... hehe

)
etc. Kinda vague but ....
Hope that helps a little,
DanMan