How i can make that people enter in my game wear custom clothes ?

Make a script in serverscriptservice and then insert the clothes you want (Using insert service) and remove their clothes. I will not make a script for you because you need to learn something from this :)

THIS METHOD REQUIRES HTTP TO BE ENABLED

Enable http by going to Home > Game settings > Security > Allow Http requests

here you can learn more about insert service

here you can see what you need to use to insert the clothes

Btw when you insert something through insert service it will be inserted as a model so you need to get the shirt/pants out and place them in the players character