Scanning depends on a few things (OS, drivers). Did you install the full-featured software or the generic driver from the OS? (See the HP support site: http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?lc=en&dlc=en&cc=us&product=90764&
for drivers/software.) If you have installed the generic driver and a full software package is available, uninstall the driver, unplug the all-in-one from the computer and install the full software. Then reconnect the data cable.
Place the document to be scanned face down on the glass. (Document feeders need the document face up with the top edge entering the feed first.)
Depending on the setup, you can launch the software from your computer. Double-click on the icon on the desktop or (in Windows) Start > All Programs > HP folder and then click on the HP Director. (Once this software is set up for the scan destinations, if the monitor program is running, you'll be able to press the scanner button on the all-in-one to launch the scan. This is under Settings) Once the program is running, click Scan Picture or Scan Document depending on the item to be scanned. Adjust resolution and other settings as needed - try the default first. Click Scan. If doing a preview scan then click Accept when satisfied with the settings.
If you do not have the full software available for your OS or simply do not want to use that software, you have some alternatives. Windows XP, Vista and 7 all can acquire scans in Paint. (Open the program, then File > From Scanner or Camera) Some versions of Vista and Win7 included Windows Fax and Scan which also acquires the scans (just click New scan). Vista has another program that can acquire scans (Photo Gallery IIRC). Macs may have similar software available. In both cases, third-party software is also available.
If your computer doesn't see the scanner, check the cables. Scanners require bi-directional communication with the computer so a cable that allows printing can be a problem with the scanner. Try a new cable or plugging in to a different port. If you have a USB hub or print server, try a direct connection.
I hope this helps.