Offers are a great way to drive new people to your business and to engage your customers. They also encourage people to talk about your Page when your offers are shared between friends. Best of all, they are free for admins to use!

How do I create an offer for my Facebook Page?
To create an offer:

1. Go to your Page
2. From the sharing tool at the top of your Page's timeline, click  Offer, Event + and then click  Offer
3. Type a strong headline for your offer, ex. "Buy one pair of socks, get another pair free."
4. Upload a photo for your offer that will stand out in thumbnail size
5. Choose to limit the number of claims by clicking Unlimited and selecting a number from the dropdown
6. Set an expiration date by clicking today's date and clicking a different day in the future
7. Add terms and conditions of your offer
8. Click Preview
9. Review what your offer will look like, then click Post

If you want to make changes to your offer when you're previewing it, click Edit to go back and make changes. 

For more details see: https://www.facebook.com/help/offers/