1. Volunteer is not something that you must do.
2. Volunteer is not anything that you must do.
3. Volunteer is not a thing that you must do.
4. Volunteer is not the thing that you must do.
5. Volunteer is not an important thing that you must do.
(What is the difference between #1 and #2? Does # 1 mean #3, 4 or 5? Thank you.)
Rather than using "Volunteer" in each of those sentences, you need to use the gerund form: "Volunteering."
The sentences' meanings are similar. #2 has a slightly more negative tone to it.