What is wrong with my film camera?

I bought a used Nikon FM-10 for $40. When I shot my first roll of film, at the 12th frame the viewfinder got all black so I was like how do I fix this so I tried pressing the dept of field lever and the black went away and it took the picture (the shutter released). All frames after that the same thing happened and I continued doing that till my last frame and I rewinded the film and unloaded it for developing. It turns out that all pictures before the 12th frame was fine but after that the film ripped and was unprintable. So I thought that it was because I didn't load it correctly but I did everything that I was shown how. Today I shot with B&W film (haven't developed yet) and the first couple frames were normal but later on it happened again but this time it was every other frame that it happens. When I was done and about to rewind the film I accidently pulled the rewind knob up and it opened the door and my film was exposed. Do you think all frames were ruined or only 2-4? I'm going to get it developed anyways because I can do B&W for free at school. Do you know what's wrong with this camera; is it something that can be fixed? It's really weird because sometimes it happens and sometimes it doesn't. The pictures I took were ducks at the park so I wish I didn't mess it up.

Added (1). And could it also be because I didn't pull the film advance lever all the way?

Added (2). Sometimes when I pulled the advance lever it felt like the film didn't advance smoothly so there's definitely something wrong in there.

Added (3). Yes I used the light meter. I've used digital cameras for a long time already so I understand exposure. I'm going to give the camera to my friend who knows how to use it and see if he get's the same problem. Hopefully this camera works or I'll have to look for another one.

This sounds like a problem with the winding mechanism. Try this: buy a cheap, out-of-date roll of film and run it through the camera with the back open. That way, you can see what is causing the mirror to stick and the film to jam. Or it may be that you're just not advancing the lever all the way when you wind on.