1. Don't touch your face.
2. Cough into your elbow, covering your mouth and nose as completely as possible.
3. Every time you shake hands, touch something in public, sneezed, coughed, etc., wash your hands for 20 seconds and be sure to scrub fingertips and knuckles, as well as palms. Using plain soap and water is sufficient, and this twitter thread explains why:
CDC tips for hand-washing: https://www.cdc.gov/handwashing/when-how-handwashing.html
When scrubbing your hands, there are some right ways to do it. Here's a helpful video: https://www.youtube.com/watch?v=seA1wbXUQTs
4. Last of all, if you can't wash your hands, scrub with a squirt of hand sanitizer that is at least 60% alcohol. Or make your own. There are many home recipes on the internet.