Image courtesy: http://www.bitcoin-exchange-berlin.com/2013_04_01_archive.html (Photoshopped Image)

In 4 simple steps.

1. Visit http://bitcoin.org/en/choose-your-wallet and select a wallet that suits your requirements.
2. Once you get a Bitcoin address, your account has been setup. This process is generally completed in 2-3 minutes.
3. Print out the image below and stick it on your shop window near the cash counter and outside your shop.

Image Courtesy: http://hostquickly.net/pages/wp-content/uploads/2013/10/bitcoin_accepted_here.png

4. Accept Bitcoin using the address created from the wallet of your choice.

Disclaimer: Do read a bit about Bitcoin before diving in.