Close the door/window when the Snowman gets near. You can check the cams, but the cams have a blind spot: your room. Yes, it is dumb, but this is a horror game. Deal with it. You also have to sleep. If you don't sleep, the night will never pass by. (Disclaimer: There are sound cues which let you know where he is, but you only hear footsteps when in bed. ;)) Controls: Click door to go to hallway. Click window to go to window. Right arrow key to change cams. Click when at window or hallway to close window/door. Click bed to sleep.
Inspired by FNaF. The Thumbnail code thanks to @tsfcar.