Post a new thread under the Forums -> Appeals -> (Server), whether it's .Net, .Org, or Hellblock. Typically, the moderators require that you use the following format; In the title write: [Your in game name] Reason for ban [The Person who banned you] Server name (.net / .org ): In-game name: Full exact Ban Message: Did you read the rules ? Why do YOU think you were banned? Was this an incorrect ban? Were you ever banned before on SkyBlock? If so, what for? Why do you think you should be unbanned? Also, read up on the rules to be well versed in them. Good luck.
Then wait it out, or comply with the moderators' rules. My understanding is you're not to post outside of your appeal, and using alts is a bannable offense.