I am wiring a light switch at the end of a circuit. I have a junction box, which the source wires enter. I have a wire running from the top of the switch back to the junction box and another wire from the bottom of the switch also going back to the junctions. Which wire get connected?

None yet there isn't enough information here that's really a complete question
Connected to what! And its not another wire if what in the box so far it's a loop of nothing
Missing a lot here to answer

Let's see if we can get this working for you.

The cable coming from the receptacle will be the feed to your switch.

Turn power to the circuit off at the main panel box.

Black will connect to the bottom of the switch. White will not connect to the switch.

You need to run a cable to the from the switch outlet box to the overhead (?) light's outlet. Black will connect to the black fixture wire. White will connect to the white fixture wire.

Back at the switch, connect black from the fixture run to the top of the switch. Connect the two white wires together. Now go back and make sure that all equipment grounding conductors are properly connected.

Then go back to the panel and turn the breaker back on and test the new circuit.

Joie,
I am with Boat. I am totally confused by your post.