Hi ClulessGirl56,
Welcome to the Support Communities!
There are actually three ways to do this. One is to change your preferences in your iTunes application on your computer to Automatically download anything you purchase. See the link below for instructions:
iTunes Store: How to enable Automatic Downloads
http://support.apple.com/kb/HT4539
You can also download anything you've purchased from the iTunes Store to your computer at any time:
Downloading past purchases from the iTunes Store, App Store, and iBooks Store
http://support.apple.com/kb/HT2519
And finally, you can transfer purchases from your iPad to your computer when you backup your iPad.
iTunes Store: Transferring purchases from your iOS device or iPod to a computer
http://support.apple.com/kb/HT1848
Hope this helps ...
- Judy