How to switch players turn in online game?

So I want to make a online turn based game.When for example player1 shoots its player2 turn.But since its an online i think that it would be much harder than that.If someone has experience such kind of games or knows how to do it ,please share or give some hints of how to do this in an effective way.I don't want to write 200 line of code that can be done with 15.