How do I begin finishing my basement?

I just need one small room to feel comfortable enough for an office space. The area I want to finish is already a separate room. The basement looks really really old. Built in 1900. The floor is concrete with a slope in the corner where there is a drain. But there's no leakage down there. The ceiling is about 7 feet tall with the pipes and wires and cobwebs and stuff. The walls are brick. What do I do? How do I find out how to do it, and most of all, how much is it gonna cost me?