How do you respond to rude comments.?

i was wondering here in nyc ppl are always arguing on the bus or grocery store or w/e. and ppl are really rude. whats do u do if someone said STFU and they were really extremely ghetto. would you just do it and walk away and look like a weakling? what if someone said something childish like "go fix your crooked azz teeff " or "your girlfriend is an ugly fat btichh" i heard em all here especially in high school one guy told another "suck ma " and they started fighting like apes.another time some girl accidentally step on another girls shoe and they started fighting and hair pulling i mean wtf is up with todays youth.these kids are the future? lol