How do I trigger a condition switch?

I am trying to create a conditional event wherein the pc will die/ receive a game-over when their hp=0.
I do not want to use the 'battle mode' that comes with RPG maker, rather when the pc runs into an object and looses health to the point where their hp =0, they will die.

How do I make the pc's hp a variable? (or how do I set a condition switch?) How can I use this to trigger a common event?
Thanks y'all

I suggest you follow the link to the starting point in my signature and work through the tutorials linked there, that will teach you the basics as fast as possible.
Especially the links to event tutorial games and to the variables guide game.

Those tutorials were written for VXA/MV, not for XP, but variables and events are really basic and haven't changed much since RMXP.

Thanks very much for the help!!

Doesn't the game end when the player's HP reaches 0 by default? Even outside of battle?

If that isn't happening for you, then you may have something set up in the System tab that prevents it, or a plugin that prevents it, or you may not have set up your starting party members correctly. If you only want one character in the party, remove the others from the starting party in the system tab, rather than deleting them from the actors tab, which would leave you with three "ghost" party members.