I want to make a online undertale RP game but I have no idea how to make it online…

Making an online game takes alot of time and code. I cannot explain how to on Scratch forums, but you can resort to other sources on Online Scratch games or watching a tutorial. Here are some games that can help: https://scratch.mit.edu/projects/409133816/
https://scratch.mit.edu/projects/450327605/

do this for the y position too and on the next sprite have got to variable if player = not what that sprite is set to