If you shopped at Target in the United States between Thanksgiving and Christmas then there is a pretty good chance that your credit card data for the credit card that you used was stolen by some cyber thieves. Because of this massive data theft Target has arranged for credit monitoring services for all Target customers for a year.
The only way to get signed up is via the webpage which target has setup at https://creditmonitoring.target.com/. Any other website which claims to be signing people up for this (and there are going to be several of them popping up rather quickly). Also if anyone calls you trying to get you signed up they are scamming you.
If you shopped at Target, I’d recommend getting signed up for the monitoring service. According to Target:
Guests have until April 23, 2014 to sign up to receive an activation code. Activation codes must be redeemed by April 30, 2014.
So go get signed up,