For that kind of lock, call a local locksmith shop or if cabinet is movable, bring it to a locksmith shop who will more than likely have a key that will work on his wall of keys, or can at leaat open it for less expense at their shop than driving out and charging you for doing so.
It is not an overly complicated mechanism, but might as well be impossible or very difficult if you are a total beginner and have never picked a lock. It is not the kind of mechanism we here normally pick with our lock picks, so trying to convey how to open it via instructions on the forum would be like teaching you how to paint by reading a book with no pictures.
One other possibility is contacting this site and showing them your photo and some keyhole measurements. They may have what you need.
http://www.antiquekeys.net/keyblanks.htmlSquelchtone