This depends on a lot of things, mostly what do you want this robot to do? (ex. Drive forward a few feet, react to sensor inputs, drive autonomously, etc.)
If you intend on doing a fully autonomous system, well... Good luck, I am in the process of building a rocket guidance system, that has only 1 axis of control, and it already is extremely difficult.
If you just want it to do simple things, like stop when a sensor says something is there, or follow a line, or just follow a pre-programmed course this isn't so bad.
C++ is a great language, it can be used for almost anything, and many micro-controllers use it. You do not need to know assembly at all.
Even if you are building an embedded system from scratch you still don't really need to know assembly. C is very similar to C++ though C++ has a few syntax changes, and some extra features.
So give me a few more details and I should be able to give you a more accurate assessment of what it's going to take.