Developing a multi-player game - How should I approach this?

Recently I have started to work on a multiplayer game in Libgdx with the help of AppWarp. Now, this is my first time in the multiplayer scene. I have developed several games in Libgdx but none of them had a multiplayer feature.

From a general aspect, how should I start working on it? Should I start with the core engine(the game itself) and then adding the multiplayer or should I do the opposite?

If I may, I would like to hear some experienced developers who have experience with such game Smiley

P.S The game is going to be in real time. It’s some sort of a race game.