What should i do if i'm self-quarantined with someone who may or may not have covid-19? how do i avoid catching what they have?

The basics: I would wash hands often, keep a tolerable distance & wear masks. There is emerging evidence that a mask can reduce spread by droplet to negligible amounts. Since as many as 50% may be asymptomatic & spread the virus throughout the illness you may have been exposed anyway. If either of you develop breathing difficulty/shortness of breath get evaluated.