Hi, are you sure there's not a cutoff valve tucked away behind a removable panel in the bathroom? Maybe he put it in the same place as the cutoff valve to the bathtub/shower? It seems very unusual that there's not a cutoff valve in there. The house my mother-in-law lives in has had water lines run since the house was built, and the cutoff valves were in the water heater room...it took us about 20 minutes to figure out what the different cutoff valves controlled (none were labeled at first, but they are now!).
That said, you should only need an in-line valve that matches the size and type (PVC, copper, etc.) of water line feeding your sink. You'll have to cut the feed line as cleanly as you can. Since you'll probably have a solid feed pipe, you'll end up soldering (copper line)/gluing (PVC) the valve into the line. You hopefully have good access to the feed line at the location you want to install the valve. You'll want to make sure that it's installed after any other junctions that water feed line has in it (to make sure you're not cutting off the water supply to other sinks, etc.).
A good do-it-yourself home repair book should have detailed instructions and pictures in it. I've seen them at Home Depot and Lowe's stores. The Black & Decker series had a book dedicated to plumbing, and I know Readers' Digest also sells a fairly comprehensive home repair book (I own both). Good luck!
Jim D/Heathsville, VA