How can check who's looking at my screen and camera?

somebody help me someone has been blackmailing me saying he has videos of me and that if will send them all to my contacts he said that he got those after accessing a site i logged in to he said he can see my screen and camera at all times what should i do is there a way i can check whos looking at my screen and camera? Please somebody help me

[Re-Titled by Moderator]