Write the President, your two Senators, and your Representative, telling them what you want them to do. The simplest way to find their contact page: Google "contact <person's name>". That's what I did, and here's my message (if you want to crib from it):

-- begin -- I strongly support you doing whatever you feel best about to increase Americans' safety against gun violence. I would be in favor of:

* limiting access to guns (especially assault weapons)

* increasing access to mental health care

* increasing gun owners' legal liability for actions performed using guns registered to them

Thank you so much for your leadership and work on this.

Respectfully, <your name here

-- end --