Start with C or C++. K&R is always a handy starting reference. Learning a typed language I believe is harder than something like C#, VB or Java but that is just me. You will never become an expert at any language and you really should not try to. If you are really good you actually learn how to learn languages rather well. If you want to do kernel and driver work like you hinted at then you will need C. Most kernels do not support OO so C++ and managed languages are out for the most part. You can still use C++ for then in some cases you just can't use certain parts of the language like OO.

If you have been in the industry before and are looking to join a software company like M$ or others they will typically not ask for samples. If you get to the interview you will have to demonstrate in real time your skills.

It is a big red flag if a company asks for samples before hand. It usally indicates you are interviewing at non tech company or they have inferior managers whom can not weed out candidates well.