Zandronum

I really want to make a invasion wad but I know how to put custom monsters now but I have no clue what to do with skulltag acs or anything plus I want to make the first wave stop from responding after I kill all the monsters an I kill the first wave an the 2nd wave an I want to know more about skulltag acs features an more cool things that you an I seen in invasion wads plus I want to know how to make doors open after the waves end for example I kill 5 waves and the door opens all by itself pretty much I want to know everything say what you want about this topic

Since you're at a beginner level, I recommend using the old skulltag invasions spot actors. They are documented on the ZDoom wiki.

A proper solution would require extensive ACS. I did this at one point as a possibility for INVULED:EW, but it was never really finished. I do not recommend this route due to your lack of experience.