A new kitten needs time to settle into their new surroundings. During their first weeks in your home, they should be allowed some quiet time to explore each room thoroughly. Only after they have grown accustomed to their indoor environment and have adjusted to a regular feeding schedule should you allow them to venture outdoors. If you acquire a kitten during the winter, you should wait for warmer weather before allowing them outside.
Whether or not your kitten should be permitted access to the outdoors is an individual decision. Although they will have more opportunity for exercise, they will almost certainly be exposed to many more dangers, such as traffic and dogs. Whatever your choice, you should decide early on; once a cat has been given outdoor freedom, they will not easily be confined thereafter.
If you have chosen to give your kitten access to the outdoors (and if your home permits it) the addition of a cat flap can be a great convenience.