Help making a main menu for a 3d game

I have tried to find clear information on how to program a main menu for my game, but I have found nothing. I also tried to use other existing games as an example but when I go into editor I cant even find the ui scene or entities. I have tried several codes but non will work. If there is a clear tutorial on howto make a ui that will just change from one scene to another, then that would be amazing. I have searched for a week now and I really can´t find one. My game is in 3d.

Try these tutorials: (the non-HTML ones will probably be bet for you).