Unfortunately there aren’t any guaranteed steps. If the panel is in alarm state it absolutely has to be disarmed with a code to get started.
If the old owner left it in an alarm state and did not provide a code it is likely not usable.
A similar question is found below.
If the original installer did not lock out factory resets (there is a setting which does this, and is often locked) then you might be able to reset. I have never tried while in an alarm condition, I doubt it will work, but worth a try.
Power down the panel completely, hold the two face buttons, and power up (don’t need battery). Keep holding the buttons for 60 seconds, then release. Upon power up, try 1111.