Thermostat says lowbat.....where is the battery and what kind of battery do i need to replace? will my furnace still come on?

If you replace the battery in time, the furnace should still work. You didn't mention what type thermostat you have, but the battery is located inside the thermostat housing. Most can be accessed my opening the flip down cover of the front of the thermostat (this where you generally program it). Inside you will either see instructions for replacing the battery or you will see another small compartment door with a battery icon on it or it says "Battery". If none of that appears as stated, the batter will be behind the thermostat housing. It's generally 1 AA Battery.

Hope this helps you solve the problem.