My mom's Boyfriend has fucked me over and changed the password to the router and shit, but he has the password entered in his computer which I'm using right now. How would I get the password?

You can't, unless you can find a copy of it that is unencrypted (for example in some web browsers you can view stored passwords).

The other option is to reset the passwords on the router back to their factory defaults. Then of course change them to something of your choosing.