Depends on your level really.
If you’re a low level I’d suggest killing low leveled monsters and then selling their drops to a nearby General Store. (Don’t forget to bury the bones, prayer comes in handy.)
Alternately: Regardless of level you can skill (Woodcutting, Fishing, Mining mainly) and sell those items on the G.E.
If you want a better guide, RuneHQ might have something like that.