how do I print a receipt for an app from the app store

You should have received an email with a receipt for the app you bought. The email will go to the email address on your Apple ID. You can print this receipt from the email however it make take up to 24 hours to appear in your inbox.

You've checked the spam folder as well as the inbox on your email account ? Mine quite often don't appear until sometime the following day. You might be able to request a duplicate email via http://reportaproblem.apple.com

After each purchase, you get a receipt sent to your email address. You can print the email and attach it to your records.

I think you can also locate your purchase in iTunes (open the iTunes Store, click Account, and then select Purchase History) and take a screenshot (like this).