How do I keep my puppy warm in the winter?

i just got a puppy. his name is oliver. he is eight weeks old. as you know, the weather is getting colder now that it is december. it has even snowed a few times. how do i make sure ollie stays warm when i put him in his crate at night, or when i leave. also how do i keep him warm when we are playing outside in the snow or cold. sense puppies can't talk i can't tell when he is too cold or too warm. please help!