It variates from server to server. Some servers are closed for the big crowd, for others you just have to send a nice e-mail to the owner or have to have a friend that gets you whitelisted. To get white-listed in the World of Minecraft SMP server, you need to be artist + in World of Minecraft:Realms (the classic server) or have bought World of Minecraft premium or get someone to sponsor you (He or she will then be responsible for your actions.)

No need for further responses, as this is solved.