Can anyone identify this tank? I'm looking for instruction to this set. I also found a strange door.
The picture of the vehicle isn't very clear. It appears to be dark green and missing several pieces. This makes an already difficult to identify set nearly impossible unfortunately. The dark green color and apparent military theme strongly suggest this isn't a Lego brand set, as already mentioned by Henrik.
What is left of the vehicle looks like some sort of APC or IAV possibly. I spent some time searching and couldn't find building sets with the Marine Corps sticker on them. It is very likely this was made by one of the many companies that make Lego compatible military sets, and even if we knew the exact set, it is highly unlikely that building instructions could be found. To try to help you figure out what's missing on your set I've included a couple pictures to give you an idea of what your set might have looked like.
As for the door, very little can be determined from the picture.