To do this all you will need to do is push the Home Button and the Power Button at the same time. The screen should then blink. Your Screenshot will be saved with all of your other pictures under the Saved Photos album.
Screenshot from my iPod Touch
Screenshot shown Saved Photos Album
Now to take your Screenshot off of your iPhone/iPod Touch and onto your computer you will first need to plug your device into your computer. You will then need to open the device to view your photos. In Windows you will need to goto My Computer then select your iPhone/iPod Touch to veiw your Screenshot.
iPhone/iPod Touch shown in My Computer
Now once you have it open you can now copy and paste the photos to where ever you like on your hard drive. Now you can send others all of your really cool Screenshots to all of your friends and family.