Most ultrasonic sensors you buy will be analogue. This means it will output a voltage between two values (such as 0 to 5v) which can be linearily equated to a range by scaling it appropriately. If your sensor is from Maxbotix, everything you need is
here, and if it isn't, the same concepts typically apply.
[in the event that serial sonic sensors and analogue sonar rangefinders aren't the same thing, my apologies]