Some weeks ago I participated as a volunteer in the Environmental Action Days for youth in Finland. The idea of these days is to show the participants (16-21-year-old students) how they can influence, change and work for the things that they care about.
The organizations organizing the event and/or presenting what they do during the event where:
Luonto-Liitto (The Finnish Nature League)
Natur och Miljö (The Finnish Society for Nature and Environment)
Maan Ystävät (Friends of the Earth)
Almost all of these organizations have youth-programs in place so if you are interested in volunteering in any of the organizations it's worth checking them out and sending them an e-mail or signing up for any of their activities.
If you are already older, contact them anyway! There is always something you can do.
And if you don't live in Finland, check out your local WWF/Friends of the Earth/350.org/other organization and contact them if you are interested in meeting others who care for the environment and do something together with them for the cause you care about.