How do I make a boot disk for my new iMac?

Make Your Own Mavericks, Mountain/Lion Installer

After downloading the installer you must first save the Install Mac OS X application. After the installer downloads DO NOT click on the Install button. Go to your Applications folder and make a copy of the installer. Move the copy into your Downloads folder. Now you can click on the Install button. You must do this because the installer deletes itself automatically when it finishes installing.

2. Get a USB flash drive that is at least 8 GBs. Prep this flash drive as follows:

1. Open Disk Utility in your Utilities folder.
2. After DU loads select your flash drive (this is the entry with the mfgr.'s ID and size) from the leftside list. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed.
3. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list.
4. Click on the Erase tab in the DU main window.
5. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, check the button for Zero Data and click on OK to return to the Erase window.
6. Click on the Erase button. The format process can take up to an hour depending upon the flash drive size.

Use DiskMaker X to put your installer clone onto the USB flash drive.

Make your own Mavericks flash drive installer using the Mavericks tool:

You can also create a Mavericks flash drive installer via the Terminal. Mavericks has its own built-in installer maker you use via the Terminal:

You will need a freshly partitioned and formatted USB flash drive with at least 8GBs. Leave the name of the flash drive at the system default, "Untitled." Do not change this name. Open the Terminal in the Utilities folder. Copy this entire command line after the prompt in the Terminal's window:

sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume

/Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Mavericks.app --nointeraction

Press RETURN. Enter your admin password when prompted. It will not be echoed to the screen so be careful to enter it correctly. Press RETURN, again. Wait for the process to complete which will take quite some time.