Do not factory reset the alarm system!
The system will be programmed to recognize all it sensors (doors, window, smoke and heat detectors, etc) and to correctly identify them on the control panels. For example if a kitchen window is open, the panel will say "kitchen window open" instead of just "zone 23 open". It will also be programmed to recognize "entry doors" so when you walk into the house through an entry door, you get some time to turn the system off, but not if you break in through a basement window. If you subscribe to a monitoring service, it will report fire, intrusion, etc according to the device type. If you factory reset the system you will need to reprogram everything. While it's possible to do that from the keypads, it will take you weeks to learn and do. The alarm installer will have done it from a laptop using proprietary software. You should not undo all that work!
Just learn to do what you need or get your new monitoring service to do it
You need to find the manual for your system. There is an "installer code" you can enter while the system is disarmed. It is almost certainly the factory default installer code, or if it's an ADT system, the default one that ADT uses. That will allow you to remove all user codes and to program a new "master" code. You need the installer manual to figure this out. Once you have a new master code you can then do everything you need from the homeowner's User Manual, which you should also get. And if you want monitoring, your new monitoring service will want to test and verify everything so they can probably do all this programming for you in seconds when they visit.
The battery will be somewhere in the house in a metal box, probably one with a key lock (probably not locked) and with lots of wires coming out of it. It looks like a tiny car battery. Look at the model number, you can buy them on Amazon. Meanwhile since the alarm is useless to you now, if you find that metal box you can unplug it from AC power. The battery will die completely within about 12 hours. The alarm system will beep and annoy you while that happens, but then it will leave you alone until you are ready to deal with it.