How can I make my instrument louder?

There are two main forms of acoustic amplification; viz, resonance and radiation. Both serve to take the instrument's initial vibration and amplify it manyfold. I recommend taking a read at the section on radiators & resonators in Bart's Musical instrument design book

Radiation

eg Piano sound board, kalimba base

Radiators tend to be a 'relatively' thin board with a reasonable surface area, they take the smaller amplitude vibration of the instrument and, when correctly coupled say through a bridge, turn that into a larger vibration. Radiators are useful when covering a wide range of frequencies in one instrument. The design of an efficient radiator is a complicated matter but simple radiators are readily made.

Resonator

e.g. tubes below the keys of a marimba, gourds under a balafon, the bowl of a djembe

Resonators are part sealed containers that are either cylindrical or globular (helmholtz), the container encloses a volume of air that corresponds to a specific frequency and will amplify that frequency many times. Resonators are super efficient but will only resonate a fundamental frequency or a limited range of harmonic frequencies. With cylindrical resonators the length determines frequency and the width determines the amplitude (volume) with wider tubes being louder. With a globular resonator there are no harmonics and only the fundamental is resonated, the size of the opening is critical to determining the note that is to be amplified with a smaller hole giving a lower note. I have often tuned tubular resonators by length and then fine tuned by adjusting the opening size.

Combined resonators and radiators

e.g. a violin body, an acoustic guitar body

Combining the two resonator types permits the two types of amplification to reinforce each other, the efficient combination and the factors in achieving it is a complicated business that is beyond the current scope of this website.