This week, picked at red spot left after pimple went away weeks ago. now red scab thing there. what to do to prevent scarring?

Don't pick at it: Best thing for now is not to pick at it. The more you pick the greater chance of leaving a scar. At this point, still unlikely to leave a scar. If one does form after a few weeks, then you can apply scar treatments like silicone sheets or gels that can help minimal scars. Also, can get laser treatments by a plastic surgeon to diminish the scar if one forms.