'91 4.3L Coolant leak

I noticed my coolant reservoir kept emptying after a short drive so I looked under my truck and saw what I believe is coolant dripping out of a threaded hole thats coming from somewhere near the transmission. Is that my problem and where can I find whatever needs to go there? I'm sorry if this is really vague, but I really don't know a whole lot about this all.

Could be from anywhere forward as the wind blows it back while driving. Usually will leak worse just a few minutes after startup. Try parking on concrete and starting it up and let it run for five minutes, shut it off and look for coolant on the concrete to find whether it's in front or rear of the engine. Then look closely, for steam, wetness, etc. from the leak. Many things it could be for sure.

well I just got the oil changed and they checked it and said it was probably the heater core, how much do those usually cost?

Heater core is not expensive, say 35 dollars, but a chore to install. Usually if this is leaking the carpet on the passenger side will be wet and you will get steaming of the windshield with the defrost on. Might take a close look at the smaller hoses that go into the heater core right at the firewall. Feel around them for wetness.