Zandronum

I was just wondering, how can one make a game mode for Zandronum? I wanting to make something like Conquest in Star Wars Battlefront, as far as the objective goes. How would I start?

You'll want to learn ACS and probably DECORATE. Consult the ZDoom Wiki for answers to (just about) all your questions.

Alright. I already know a decent amount of decorate, but I haven't messed with ACS yet...