That all depends on your fish. Most like it around 73-76F, from what I've read, but there are some fish that can live happily in much cooler or much warmer temperatures. You should Google info on the fish you're interested in and see.

Tropicals like it between 74F and 82F, get a glass thermometer that floats with a suction cup. You can monitor both this way to set your heater. Most have a target area highlighted in green. If you want to be precise use 1 near the bottom and one near the surface. Don't trust the "sticker" type ones.

Mollies do better in slightly cooler waters, about 75*F but are pretty adaptable. Mollies really do best in a brackish set up (mix between fresh water and salt water). Have you given any thought to adding salt and other brackish tankmates?