I will assume that you have already read this page: http://www.societyofrobots.com/sensors_sonar.shtml
It really depends on which sonar sensor you choose to use in your project - some (the PING))) sensor by Parallax for example) use 1 I/O pin for both triggering the sensor and reading back data. These tend to output a pulse whose length determines the distance detected.
Others such as the MaxSonar range, have multiple output choices such as pulse width (the same as the PING))) sensor), analog voltage output (the same as the analog SharpIR sensors) or digital serial output.
If you already have experience using the SharpIR range, then the MaxSonar range would probably be the easiest integration, as they can use the same type of interface, thus requiring very little code adjustment.
Good luck with your project