how do you make multiplayer online

You can't mate, either that or it's extremely complicated so I wouldn't dare even try… You could POSSIBLY see if it's on the wiki or on YouTube… Those are most likely the best options.

i thought it might be immposible….
but someone has done it before I know that

Yeah, griffpatch, but he's like the best scripter on Scratch ever sooo….

It's not Impossible, It's quite easy actually all you need to know is how to use Could data.

You need to make cloud data variables for every players x and y so when they move the arrows it updates the position of the sprite on everyones screen, to learn more go here: http://wiki.scratch.mit.edu/wiki/Creating_a_Multiplayer_Game