Well, for 10 minutes I was looking for how to get onto a private server.
You will need to follow these steps in order to login to the server.
1. Go onto your preferred private server site and register an account
e.g. username John_Smith and password smithismyname.
2. Copy the realmlist e.g. set realmlist smarty.ownidge.com or 18.104.22.168.
3. Since I have a mac I'm not to sure how to get to your realmlist. But for Mac, you go to the Windows Explorer and click "Hard Drive (C)" or whatever your HDD letter is.
4. Then open program files then scroll down to the bottom and you will find the "World of Warcraft folder" - open it.
5. Once inside the WoW folder open the notepad file called realmlist it should be the .WTF file.
6. Paste the realmlist into the space (Note: Don't change the patchlist, it stays the same).
Then open WoW and enjoy your time on private servers!