There is only one legitimate website to do this and it iswww.annualcreditreport.com. Beware of ads or other websites that offer a free annual credit report. Many of these are scams or they are trying to sell you something.
At www.annualcreditreport.com, you can request a free credit report once every 12 months. This credit report does not include your credit score, but you can purchase your credit score by contacting one of the three credit reporting agencies (Equifax, Experian or TransUnion).
Another great resource for credit reports is www.myfico.com, where you can purchase your credit report from all three agencies, including your credit score. myFICO also has an education section where you can learn more about your credit reports and score.
You should check your credit report annually for errors and/or fraud.