Hello. I have been playing on a public server for a long time and my first TARDIS is very boring for me. Plus, there's a monster inside, and I can't kill it. Is there a way to destroy my old TARDIS and get a new one?

You could possibly sell your TARDIS to a new player. (there's a command to transfer ownership, ask around on the server, I'm sure someone will know)

 Thank you very much for your advice. I am very glad that you responded :)