We need more information to analyze the situation. Were these checks on your bank account, but which you did not sign, meaning that someone stole your blank checks in a check book? Or were these checks payable to you, such as paychecks, and someone stole them and signed your name to the back?
Yes, you can sue Walmart.
If the checks were blank checks from your own bank account (or credit union account) then the most important thing initially is that you provide written notice to your own bank that these checks were forged. Your bank should not charge you for any transaction that was not authorized by you. The notice must be in writing and should list the checks by dollar amount and check number if you have it, as well as dates. Your letter can be handwritten, but be sure to make a photocopy of it and keep the copy. You must deliver the letter to the bank in a way that allows you to obtain proof of delivery. For example, go in person to a bank office and deliver the original to an employee and then ask the employee to sign your copy as "received" and date the receipt. Other alternatives are by fax with a fax confirmation report, or by certified mail with return receipt requested. The US Post Office has other ways to deliver an envelope where you, the sender, will receive a document that is proof of delivery.
The law is generally that the bank cannot charge your account for an unauthorized transaction, but that rule only applies if you promptly notify the bank in writing that you dispute the transaction.
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