first off. choose what will you use for MP. 3dgs native MP system or some plugins like ANet(i use it) or GSTNet. then you be sure you have enough experience with 3dgs and that you're able to do something like that.
then start drawing 2d art/stuff or hire some 2d artist and start coding.

btw i cant tell you how to make a game like that. if you're experienced enough with 3dgs you should know it by yourself