I want to replace a bathroom light switch with a combination light switch, & receptacle to run a portable bathroom heater.?

No, if there is just two insulated conductor wires and a ground then all you have is a "switch loop". The hot and neutral are run directly to the lighting j-box, neutral connected to light, and the hot is connected to a wire leading down to the switch, then the other wire from the switch connects to the black on the light fixture. No neutral is at the switch.

If you have a conduit between the light and the switch then you may be able to run a neutral from the light to the combo switch, capacity may be limited due to what else is on that circuit.

Okay.

I thought it would be simple, but there's only 2 black wires, & a ground. After installing, my light was working, the receptacle wasn't. So I had to do some research. Correct me if i'm wrong, but the simplest way is to use this infographic, & run a neutral wire up to the bathroom light. Connect it to the left bottom terminal of the receptable. That way, at least I'll be able to run the heater when the light is on. The other way would require tapping into the main breaker box, & I'm not qualified for that.

Probably depends on how many amps the heater will draw and how many lights/receptacles are on that circuit and the breaker/fuse size of that circuit?