Is the wiring at the box exposed, in conduit, behind sheetrock or plywood? Is this on the interior or exterior of the shed? Is this a finished shed, does it have a dirt floor floor? Is the environment in which the circuit in considered a dry environment or damp/wet environment?
The basics of the wiring, based on what you have described is that you have a non-GFCI protected circuit entering into the shed that is then protected by the GFCI receptacle. You can run conductors in conduit if exposed or subject to moisture, or in NM-B (romex) if not exposed (i.e. behind sheetrock, plywood, paneling, etc.) to a single gang box for a switch and then from the switch box to a round box for the light fixture. Pigtail the conductors at the GFCI receptacle and connect the receptacle, pigtail the ground at the switch and connect to the grounding screw on the yoke of the switch, wirenut your neutral conductors together at the switch and connect your line wire and switch leg wire to the switch.
Pretty simple, but it is important to understand the environment of installation so that the installation is done to current NEC code standards. Your conductors throughout the circuit MUST be rated to the minimum ampacity of the circuit protection device (fuse or circuit breaker) that they are connected to.
Based on the vagueness of the question, there are a lot of things that must be taken into consideration in running the circuit.
