Sheldon's article on caliper brakes will help you: http://www.sheldonbrown.com/calipers.html

You need to measure your reach in order to buy the correct brake. You also need to determine whether your fork is drilled for a recessed nut (probably not.) If not, you need to buy a brake with an external nut or drill out the back of the fork to accommodate the recessed nut. All of this is covered well in the article.