Cannon Rebel XSI question?

So I don't know a lot about camera and my question is pretty important but I'm sure it's something easy to fix. I have a Rebel XSI and it's a great camera the only problem is that it does this sorta auto focus thing where it focuses on one object in the background and makes everything else blurry. When you make stop motion films and you move things around the camera will focus on one object but not the other and vice versa. So how do I "tell" the camera to focus on everything and not just one object. Sorry but I am not that great with cameras and I don't know the lingo so hopefully you guys will be able to make out what I'm talking about...thanks.