How do I even start teaching my son not to go anywhere with strangers?

1.0 will be 3 in June (whaaat?) so I figure it's probably time to start making sure he knows not to go with the strange guy and his puppy. Only, I don't even know where to start! I don't want to make him paranoid of all adults but I also don't want to worry that he will end up in some creepers van.

Please help?