Raspberry Pi, who's on my wifi?

I am a noob with terminal commands and everything. i’m using raspbian which i have learned is linux based OS, Debian port. i have been wondering how to check which computers are using my wifi. if there are unkown computers on there, how do i access them? i’d hate to be getting spied on or something, we’ve had a stalker prob, long story.

I’m trying to get more comfortable with the terminal, so lmk know guys and thank you!