im making a game and i want to have it on steam. how would i have online multiplayer?
You mean how to make multiplayer game from basics or use SteamWorks Steam’s in-game online service?
I believe Steam would provide you with the documentation and help with that feature. I know of a game that was developed and worked fine and there join through steam feature went down because steam made an update. that was broken…
and then it didn’t work, the developers kept saying that they are waiting on steam to fix the update. So possible that is all managed by steam. Don’t take my word on it though.
also, just a bit more advice if this is a first project for you, you will need to have a steam account that has been active for over the last 3 years, and if it is your first submission, It will have to go on the greenlit page. and the minimum requirements to submit a game on greenllight. is 1 video clip of the game, 4 images of the game and $100 payment. I am pretty sure anyway.
UE4 has build in Steam works support 
Ahh, there we go then! I remember it had steam support, but I didn’t know to what level!