A good place to start is: http://www.nationalpopularvote.com/ It’s a movement to synchronize state law to elect members of the electoral college based on the popular vote outcome. Very successful so far.
Another place might be the remnants of the Sanders campaign: https://ourrevolution.com/ It’s yet to be seen if they can remain a force between elections, but it’s worth a shot.
Last, get involved at the local level. I went to town hall meetings as a teen in Largo, FL with my mom. We were typically the only citizens there. City and local governments have more power in everyday lives than DC has, yet it’s neglected by everyone. If you want to change your community (or keep it the way it is), start there.