Once they get the candidate situation situated, donate and canvass for them. Convince your friends to vote. If you feel voting doesn't matter, remember there's more on the ballot than the presidential race. A lot of the time meaningful change starts at the local level. You'll see your political power in action if you get involved in your community.
#1 priority is to get out the vote between you, your friends, family and coworkers, and take the trust that Biden has just put in you and everyone else to uphold your democratic values as a nation.
If you want a challenge, your goal is to track every bullshit reason Republicans (some in Democratic sheep's clothing) fabricate that the new nominee is unelectable, and communicate to your network why those reasons are bullshit, no matter how many times the media repeats it to pass it off as true.