I think someone is watching me on my webcam... Please help?

[Charlotte]

So 2 days ago I was using my laptop and just browsing on facebook and twitter, and my webcam kept on turning on. I have a Macbook Pro and I know when it's on because of the green light. At first I ignored it as I just thought one of my applications like FaceTime or Photobooth were just turned on. But it kept on turning off and turning on by itself, so I got a little paranoid. I quit both of those applications and it turned off then. But then the day after I turned on my laptop and the light switched on after about 10 minutes. The only application i had open was safari and i had no tabs that involved the webcam. it switched on and off for a while and then it these strange ads popped up... Nothing creepy just ads for other websites, not porn or anything. I then closed my lid and didn't turn my laptop on until this morning. I turned it on this morning and the light was on again... and i swear, but not 100% sure that this picture or video or something of a man popped up in the upper right corner. Then my laptop spazzed out like windows started opening and then I just closed my laptop screen... And it's still closed.I'm using my mum's laptop right now.

I'm just confused, becuase I NEVER download anything that could have a virus. I never go on any strange websites that could give me a virus. So i just don't understand what is happening, and I am really really scared. I never change in front of my laptop thankfully, but I use it quite a lot so I am very scared.

What should I do? I haven't told my parents or brother yet because they're out. I'm only 15 so i'm really scared. please help? Thank you so much.
Thanks for all your answers, they are all so helpful