Wow. If you are actually going to make a guitar then I must say that is quite challenging. I have a friend who is a woodworker who tried to make several guitars - but they just did not work out so well.
Here is what I can share about volume as it relates to guitars.
Let's start with an acoustic guitar (no electronics to amplify the sound). An acoustic guitar produces sound when a plucked (or otherwise excited) string vibrates the soundboard. Let's examine this in more intricate detail.
Plucking the string itself will not create much of an audible sound. So the string by itself does not produce the sound. Rather the string vibrates at a given frequency (depending on how it's tuned and fretted) which then vibrates the saddle (at that frequency) which sits atop the bridge which is attached to the top - also known as the soundboard. When the soundboard vibrates at the frequency transferred from the vibration of the string, audible sound is produced. To a lesser extent, the vibrations of the back and sides will also contribute to some sound. But the top itself is optimized to be the most sensitive to vibration induced by the strings.
Much of the sound created by the top and back and sides emanates through the sound hole, so the size of the soundhole can affect the volume as well (although you can plug the soundhole and still get a great deal of sound).
To a much lesser extent the vibrating string vibrates the nut which in turn vibrates the neck which can send some secondary vibration to the top which contributes to a sound sustaining.
The amount of volume a given acoustic guitar is capable of producing depends on many factors including the type and stiffness of the wood used, they type and design of the bracing, the shape of the guitar, the size of the guitar and the type and gauge of strings used. Other factors would even include the action or how high the strings sit above the fretboard.
On any given acoustic guitar, the volume is controlled by the velocity of the action used to set the string into motion. In other words the more forcefully you strike the string with a pick or pluck it with your finger or thumb - the more it will vibrate the top and the louder the sound will be. The thickness of a pick will affect the picks ability to excite the string (thicker picks will allow for more volume than thinner picks).
The volume can be attenuated by techniques such as palm muting or muting with the fretting hand. Either type muting can slow or stop the vibration and lower the volume.
An electric guitar depends on electronics and amplification to produce sound. The string vibration is detected by a magnetic pickup which sends an electrical signal to an amplifier which sends a signal to a speaker in the amplifier. When the speaker cone vibrates (at a frequency determined by the strings vibration) - audible sound is produced. The volume of sound on an electric guitar can be controlled similarly to an acoustic but is also controlled by volume knobs on the guitar itself as well as volume and gain controls on the amplifier.
I hope I have provided something you can use and for those not working on a music project - something of interest.