How do I made an online multiplayer kinda like smash bros where i have a certain amount of players and how do i make it work with a character selection screen and a stage selection screen?
There are plenty of example projects. I believe you’ll find one for combat, one for multiplayer, one for button combos, and one for selection screen.
You have to review them, understand them, and then somehow magically merge the features together 
But for starters, I recommend following one of the basic tutorials to get started with GDevelop.