Why for starters, check out these pages:
NParks has a whole load of opportunities listed on their Volunteer Page. The good thing is you are now required to select a home-base; with this association, you wi be notified if oppotunities near where you live. If interested in conservation activities, you will have to attend an orientation before volunteering, which wil be helpful.
Lepak in SG has organised a local directory of nature and environment groups: https://lepakinsg.wordpress.com.
Our volunteer group Facebook pages highlight opportunities offered by others in the community, so will help you find your calling!