Make sure the crate is a comfortable place for your puppy to be; it should have a soft crate pad, a water dish, a small variety of chew toys (soft & hard) that you change frequently. The crate should be sized appropriately; large crates for what will eventually be a large dog usually come with a temporary divider to make the space smaller. Leave a radio on for company. Don't use the crate for punishment and don't leave your puppy in it for too long. At 12 weeks, your puppy still needs lots of opportunities for potty breaks outside and lots of exercise and play time. If necessary, give your puppy a little biscuit to go inside the crate. In time, he'll learn that his crate is "his place" and he'll probably go in just to nap even when you're home. Until then, you'll need to work with him to teach him that.