When i was brushing my teeth my tooth brush slipped and cut my gum. a pimple has formed in its pice. what should i do?

Watch and rinse: Rinse with warm salt water and watch for pimple to recede. If it does not, see a dentist to evaluate what the lesion is.

Gum heals fast: It's from the cut, and the mouth is a well hydrated area, so it looks like a sealed off area that's appears like a pimple. Just do warm saline rinses/gargles to keep the area from getting inflamed, or infected, and within 24-48 hours it should be healed. Change toothbrush too! Most are designed blunt and do not cut gums regularly. Long fingernails can though.