Hey Jean, if you are new I would start with something easier tbh but to answer your question - I would do it with an array.
Every piece of your scenery would have a number associated with it and when you make the map locally it is saved in that array and shared with the host or peers for them to create it on their side.
A tiny rectangle map might look like this in the array:
6 / 1 / 0 / 0 / 3
4 / 0 / 0 / 3 / 5
2 / 4 / 0 / 3 / 6
Still, I would suggest you just focus on making the game without the level editor because again if you are new you will find that hard enough I think. Especially if it is multiplayer. If you can do this then adding a level editor afterwards wont be a massive issue.