Sound From Vibrations
When you strike any object, it vibrates, at least a little bit. The vibration, if it is audible to human ears (between 12 hertz and 20 kilohertz), is sound. Pluck a single human hair, and it doesn't make much noise--but it does vibrate; it's just that those vibrations don't vibrate the surrounding air in a way that you can hear. The sound an object makes when you strike it is called "resonance." If you strike a desk, it makes a dull thump. If you strike a tuning fork, it sounds a high-pitched note. Think of the dull thump as the resonance of the desk and the high-pitched note as the resonance of the tuning fork. (Resonance is actually a general term for the tendency of objects to make noise when struck; the loud thump of the desk is technically the audible resonant frequency of the desk. See Resources for a link regarding the physics of resonance.)
The Acoustic Guitar
In an acoustic guitar, the strings are mounted on the bridge right below the hole in the wooden box of the guitar. The bridge transfers the vibrations of the strings into the sound box of the guitar--the curved wooden body. When the sound box vibrates, it makes the air inside it vibrate, and you can hear that--in fact, that is mostly what you're hearing, not the strings. Imagine someone playing an electric guitar that is not plugged in. The sound is the sound of the strings alone. (Electric guitars don't amplify their sound with sound boxes that resonate; they use electronic amplifiers.)
You need an object that will vibrate in a way that will complement that sound to amplify sound. Sometimes an object vibrates at exactly the same frequency as the sound. This is what happens when an opera singer sings so high that she shatters champagne flutes. She sings a note that is at exactly the same frequency as the resonant frequency of the glass. That makes the glass vibrate violently, and if she holds the note more than a few seconds, the glass breaks. So, you need to figure out what pitch the sound you want to amplify will be (what frequency it will be) and then find an object that will resonate at a complementary frequency. It all depends on what kind of sound you want to amplify.