The battery is the most likely problem. Charge it, disconnect it overnight, see what happens.
The ammeter test is a good diagnostic tool, assuming you have an ammeter and understand how to use it. You disconnect a battery lead and connect one lead of the ammeter to the battery, the other lead to the battery clamp. With the ignition off and the doors closed, you will probably get a reading of a few milliamps. That's just the keepalive power for the computers and radio, plus any alarms, remote start devices, etc... A good battery will not run down from a few milliamps for months. If it is more, like several ampere's, then you need to look at a wiring diagram for your year and start pulling fuses to eliminate possible sources.
Now, you can do the same thing with a test light. Hook it up the same way and see what fuses make it turn on and off. The problem is that the keepalive power I mentioned could be enough to make a small test light turn on, and mislead you.
I heard of an old school solution to the test light problem. Find an old headlight bulb, like a big old 70's glass one. They are dirt cheap at the stores, probably free at a junkyard. Get some wire and use that instead of the test light. The keepalive voltage will make it glow dimly, if at all. A real power drain will make it shine more brightly. Then, pull fuses, etc...
You might charge the battery and go around and check your lights. Maybe your brakelight switch is stuck.