Double down if the dealer is showing a 3, 4, 5, or 6.
Otherwise take a hit.
Most of basic strategy assumes you'll be dealt a ten, since there are more of them in the deck than any other card. If you're holding a nine, and you get dealt a ten, then your total is nineteen.
A nineteen is a very solid hand, and combined with the potential for the dealer to bust, makes it a profitable place to double down. (Basic strategy almost always assumes that the dealer's downcard is also a ten, which is why 3, 4, 5, and 6 are probably going to make a stiff hand for the dealer.)