Q: Reset new iMac to factory settings

Do an Internet Recovery Boot as instructed at OS X: About OS X Recovery - Apple Support

This will return the computer to the original state when first taken out of the box: What to do before selling or giving away your Mac. If you just wish to re-install OS X then you have two basic options: 1. Reinstall without erasing the drive; 2. Erasing the drive then reinstalling.

Reinstall El Capitan Without Erasing Drive

       Please be sure you back up.

1. Restart the computer. Immediately after the chime hold down the Command and R keys until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the the side list.
4. Click on the First Aid icon in the toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Reinstall OS X and click on the Continue button.

Also, see this User Tip: Basic Steps for Upgrading OS X.

Install Yosemite or El Capitan from Scratch

Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the side list.
3. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

[Permission to use part or all of the above has been granted by Kappy, exclusively, to theratter.]

Both of the above assume that you can start the computer from the Recovery HD.