Trackmania is not the easiest game to produce ! But it is possible to make a networked game with Unity Built In System !
I suggest you begin by trying some multiplayer examples ! The Zero-to-Hero guide, quoted in the other answer is here : http://forum.unity3d.com/threads/29015-UniKnowledge-entry-Unity-Networking-the-Zero-to-Hero-guide This one takes some time to understand, but it is good.
To test multiplayer, you must have at least a client and a server. So you have to compile a scene, so you can have a webplayer (server for example) and a client running on the editor.