Go to a meeting (e.g. a volunteer meeting) with a climate/environmental organization.
There are many climate/environmental organizations in different parts of the country. They usually host open volunteer meetings or workshops where you can participate and help out. Go to a meeting and check out the local team. Greenpeace, 350.org, Friends of the Earth, Climate Action, Nature Preservation Society are just a few of the more well known. Go to klimatsverige.se for a much longer list.