Sounds like you're asking for info regarding a factory reset (hard reset). This will delete all apps and data you installed on the phone, resetting the phone to the original software state you received it in.
Here are the instructions from the Desire S User Guide.
1. Remove the battery, wait for a few seconds, then reinsert the battery.
2. Press and hold the VOLUME DOWN button, and then briefly press the POWER
3. Wait for the screen with the three Android images to appear, and then release
the VOLUME DOWN button.
4. Press VOLUME DOWN to select FACTORY RESET, and then press the POWER