1 outlet / 2 switches: want to replace old outlet with GFCI
Both the black wires currently on the receptacle, need to be connected together with a pigtail. The pigtail will then connect to the brass colored LINE terminal on the new GFCI.

The white wire from the old receptacle, will attach to the silver colored LINE terminal of the new GFCI.

The grounding conductors should also be pigtailed, and a pigtail terminated at each devices grounding terminal.



The wiring seems a bit strange in that box, but I'll have to think about it a bit more before I decide if it's dangerous.  

In the meantime, you should use a black marker or tape, to mark both ends of the white wire that's attached to the switch.