Just a warning that this is gonna be kinda boring if you have no idea what the game Half Life is,
but even if you dont you should def check it out (this is also not a tutorial, just sharing), its a old school fps where you fight aliens and the military,
getting into modding is not as hard as it seems in my opinion, Dimbeak's tutorial really helps you get to it when starting,
but basically you will need a copy of the game, obviously, a map editor, such as Hammer or J.A.C.K, thats where Counter Strike was made,
you will also need time and effort, cuz it takes some time to put all the little blocks together and make a map and stuff.
Image of a map I made in J.A.C.K editor
Holy shit, why do some people have it easier, life is NOT fair, can a girl even look at me once, not sounding like a weirdo.