How can you make things visible for one player and invisible for another?

Hi! I am going to be straight up and just say: I have no idea how I would script this.. I also don't know how I would go about this too. but I know this is possible because in a lot of Player customization games the other players are invisible and some games have billboard GUI that are invisible to them self and visible to others. For example a health bar. If any one you know (Including you) knows how to do this or steer me into the right direction, it would be greatly appreciated!