You listen to them and bring something related to the prior subject, when you can you use one of their expresions in a funny way so you let them know that you listen. You can talk about yourself too, most people like talking about themselves, it's pretty boring after a while, it depends on the person and how interested you are.

it helps if you have shit in common. if you have nothing in common and you're not chad, good fucking luck. some people will just blab to you about shit you don't care about for hours, though. not sure that's any better, though, because then you have to pretend to care to not be rude.