I just read your comment on my profile page, so I guess it's better to answer here than via private messaging :

What you're planning to do is a huge undertaking, and even a skilled programmer may need several years to implement what you want to do. So prepare to learn a lot, especially when it comes to 3D realtime graphics, physics, path finding and optimizations.

Zooming in from space to the "street level" of a planet is a very complex thing to implement. You need a smart (and performing) way of storing your data (even with a high-end GPU and much RAM you won't be able to keep all necessary data in memory all the time), some continuous level-of-detail algorithm with advanced visibility checks to actually only draw what's visible (again same as storing all data isn't possible, rendering everything, if not visible or too far away, would be too much for your hardware). And these are just the basic. Adding good physics is another thing and many physics engines won't be well-suited for such a continuous scaling from space to ground.

A very good place to learn about the techniques necessary for what you're about to do is http://vterrain.org

When it comes to rendering planets and all the stuff they contain this page is a treasure chest of knowledge. So maybe take a look at their articles, though most of them will be very hard to understand unless you're pretty good at programming and have done something in terms of terrain renderning.

So as an advice you should just start at ground level, render a limited terrain (using maybe octrees for visibility checks, to learn about this important feature) with basic physics that allow the player to walk on the terrain. Then start adding objects like trees and roads, include a phyisics engine for the vehicles and then zoom out stept by step, always adding to your base.