Look up “How To Factory Reset a Macbook”
Once you get to the reset screen, you will have two options:
- Restart computer (basically does literally nothing)
- Erase SSD
Erasing SSD will make it completely brand new, as if out of the shop (aside from any scratches you might have on your macbook, ofc). Make sure you back up all the data you want on a flashdrive, or multiple.