How do I delete a Team?

You mean by contacting them?...
I quess...

Or if you just want to get rid of an old team you can find an inactive player in it, transfer the leadership to them and then leaving the team.

You cannot delete a team on your own, you have to ask a moderator to do this for you. For example here in the forum.

If you let me know the name of the team (you have to be the owner), I'll delete it for you.
Note, this is a permanent action - we won't be able to restore the team, so make sure that this is what you really want before you make a request. You can also transfer the ownership to another user.