Complaining covers several different things, In Game complaining is different from complaining Out Of Game. The first option should always be In Game, whether its messaging your Ruler or Judge with your complaint or bringing your complaint to the Titans or Magistrates.
In Game Complaining
(Note: This does not cover protesting, please follow the link to the relevent page)
If you have a complaint about a person, talk to them first. If talking to them and asking them to stop doesn't work, then you should bring your complaint to your Realm's Judge (or if your personal problem is with the Judge, then you should message your Ruler).
If you feel you were banned in error, you should first talk to the Judge that banned you and explain your side of it. If that doesn't work, please see I was just banned for other IC remedies.
If and only if you believe that the ban violated your Inalienable Rights, your complaint should then be forwarded to the Titans or Magistrates. They are Tom's personal representatives on the different Continents.
If you have done all this and it doesn't answer your complaint, then accept it and move on. You have more then one character for a reason, and there is no inalienable right to be in the realm of your choice. If your inalienable rights have not been violated, you have no protections beyond those that your character can command.
Out Of Game Complaining
As with before, you should talk to the person before you do anything else. If you have done that and it doesn't work, then you should follow these steps.
This option should be used very, very sparingly. If something has happened to you, (or your account) and you feel it is vitally important for a GM, Dev or Coder to know about it or deal with your problem, there is contact information for Tom on the Battlemaster website.