Our bathroom has two bathroom sinks, I'd like to convert it to one sink. What do I have to do with the plumbing, water lines, cut off valves, drain pipe?

Basically, you will use one set of plumbing connections (h/c water and drain ) and cap off the other one.

Arec you changing the vanity or counter top over from a two tub to a one tub?
Cap off the unused water stubouts and remove all the drainage back to the wall stubout. You are now left with what amounts to a new installation. Run a "P"traqp to the drain and you're back in business. Good luck and thank you for rating this answer. Tom