How to make driving simulator environment in unity?

If you look in the asset store, Unity has a pack called "Standard Assets". In there, among many other things, is a driving simulator. Your question is a bit vague, so I'd just recommend looking at the sample project Unity Technologies provides for free.

If you want to know how to physically build the roads, you'll need to model them in a 3d modeling software, or download them online.

You have to create the roads through your 3d modeling software ex. blender(which is free to use, there are many tutorials on Youtube on how to use it), if you don't have any modeling experience I would recommend trying to shape your Unity terrain maybe for a start. Add a new terrain ad experiment with the different brushes as shown below. And for further reference you can consult the manual. https://docs.unity3d.com/Manual/script-Terrain.html