How do you get on a WOW private server and are they legal

Private servers are not authorized by Blizzard Entertainment. They could be considered illegal since the copies of the servers obviously are either hacked or stolen copies.

Connecting to private servers is against the AuP and ToS you accept every time you patch or install the client. Therefore, if you are trying to connect to a private server, then you are violating the ToS and AuP and can be banned.