There are a few ways to get a free credit report. You can sign up for a credit reporting service (like freecreditreport.com) which really means your credit report is not free as you will have to pay for the service...just your first one is free.
Some states obligate credit reporting agencies to give each person one free credit report per year. I'm not sure which states but just check with the individual credit agencies (Experion, Equifax, Transunion).
You are allowed to receive one free credit report if you are denied credit for any reason. This is usually the best way to get one for free since the main reason to see your credit report is when you are denied credit. You can then get the credit report from the credit agency (Experion, Equifax, or Transunion) which the creditor used to deny your credit application.
If you apply for a big loan such as for a car or home, usually the creditor will give you a copy of your credit report once you are approved for the loan...if not, just ask.
If you don't qualify for a free credit report there are plenty of companies out there that will provide them for you. If you are checking because you suspect some identify fraud, it is best if you get a copy from the three credit agencies named previously since different credit-granting institutions will report to these separately and you don't know where things might show up. The one thing you almost always have to pay extra for is your 'credit score'. This is a magic number computed by the credit agencies that is often used to make quick approvals or denials of credit. This number is computed using the number of credit checks, outstanding credit available, and any defaults, late payments, etc. You usually can't get those for free since the credit agencies use proprietary methods for computing their particular score.