Here is a list of organizations and networks that are doing valuable work on the climate crisis and system change:

Climate Justice Alliance (United States)
End Fossil
Soulèvements de la Terre (France)
System Change not Climate Change
Ende Gelände (Germany)
GRAIN
ATTAC
La Vía Campesina
Ecologistas en Acción (Spain)

Not everything is about large-scale action though. Building strong social networks in local communities is also important. Here are some ideas for possible projects to join:

Local Environmentalist Groups
Autonomous Social Centers, Community Centers, or projects like Community Kitchens
Radical Trade Unions
Tenants Unions
Radical Ecosocial Political Parties