How do I know how to describe my position on the airport when I contact ATC?
There isn't really an official protocol, beyond the conventions everybody uses out of habit.  Just pick the most prominent landmark, building or chart-labeled zone you can find or think of.  The controller knows all of them so as long as it's not something like "next to a light pole", he/she will figure it out, and if not, they'll ask for clarification. 

If the airport has an area designated "transient parking" on the airport chart, use that.  If you're on a ramp area close to the entrance to taxiway Alpha, you could say "xxxx on the ramp near Alpha".  If you're on a ramp in the hinterland somewhere near some hangars, pick the nearest building with a business name associated with it, like "xxxx by Standard Aviation...", or "xxxx by the Shell fuel pumps", whatever.  Use common sense and don't overthink it.