1. Don't go out unless absolutely necessary.
2. If you go out, wear the highest specification mask available to you.
3. If you go out, wear the best eye covering available to you. Sealed goggles are ideal, but even a pair of sunglasses will provide protection.
4. If you have to improvise a mask, try to use a filter material better than cotton. Some ideas: Vacuum Bags, HEPA filters, Furnace Filters, Car Cabin Air Filters. Try to make a mask that seals well against your face.
5. If you have extra masks, donate them. You may want to wait to donate them until hospitals start routinely disinfecting and reusing their PPE, because otherwise the masks you donate may get used once and thrown out. Masks donated later may make more of a difference.
Do not feel shame about wearing a medical mask out. The most useless mask is the one that sits on a shelf while its owner is outside getting infected or spreading infection. You are not denying a healthcare worker a mask by wearing one. You are being responsible and not getting infected. If you don't infected, you don't go to the hospital and infect a healthcare worker. If you don't get infected, you don't spread the virus to anyone else, and they don't go to the hospital either.
Look in your garage. Many people have a few masks left over from painting or sanding or wildfires.