Contact your personal physician or local Health Department. They can tell you what vaccinations are required and/or recommended for your trip. You will need to start your vaccinations early; some are individual doses, while others are a series requiring repeated doses spaced over several months to complete. Malaria prevention is a necessity.
Currently, the only vaccination legally required to enter Niger is the Yellow Fever vaccination; there are others that are highly recommended. If you opt to not get all the recommended vaccines, we would hope to encourage you, at the very least, to get a meningitis vaccine if your volunteer trip falls between January and May of the year. Keep in mind that the Yellow Fever vaccine is valid from 10 days AFTER it is given up to 10 years following the day the dose is given, so you cannot receive it the day you leave home.
Helpful on-line advice about vaccinations and travel: