I see heat is useful for many purposes but I have a hard time gaining it, how can I get more heat quickly?

Heat is gained mainly by walking around, depending on the location viscosity and the territory you can either gain or lose heat.

If you're talking about Heat as in Heat scars, you can do this by merely fighting.

You can gain more usable heat by increasing your briskness, sacrificing a creature, fighting, walking around (if briskness value is high enough), or releasing stored heat from your Erolin device (heat container).