Hello Lucky and Welcome

to Fish Lore. A few questions for you:
Is the tank cycled? If it "is not" cycled and there are no fish in it then I suggest adding the heater now and raising your temperature to 83-85 degrees. This will help to speed the cycle along. Once your tank has completed the cycle then you can lower the temperature to 78 or so.
Are there fish in the tank? If there are fish in the tank and the tank is cycled then I would go ahead and add the heater in as well. As mentioned it should cut on and off by itself according to the temperature that you have preset.
Also, be sure to update your profile aquarium information when necessary. It's a big help for others to know what is inside your tank and your water perimeters.
If you have the time and the means we love photos and videos

Have fun and enjoy the site. Hope to see more information about the new aquarium set up you're planning.
Best wishes!
Ken