About a week ago a friend of mine organized a call for a bunch of his friends who where wondering how they might help with the Biden / Harris campaign. I found the call very useful, but a few of my friends who were on the call said they were hoping for a simpler summary of things that they can do.
The single most important thing is to find something that works for you, and to start doing it!
Beyond that, here is a link to a list of things you can do, with lots of links to follow up on.