I think it depends on how big, how long and that sort of consideration..but in general it should be seen by a vet.
In the mean time, I would clean the sore with some mild soap and water, perhaps using a gauze cotton ball or pad, something I can throw in the trash. Then I'd use some hydrogen peroxide to further clean, again using cotton balls, perhaps trying to flush/flood the spot by dunking it into a cap of hydo peroxide. I do this every few hours and if the condition didn't improve I'd take the dog to the vet. There are over the counter animal products for minor scratches that may provide some additional help.
I say all the above assuming the condition is minor, a fact I can not determine by reading a brief post.
This site complies with the HONcode standard for trustworthy health information.
The Content on this Site is presented in a summary fashion, and is intended to be used for educational and entertainment purposes only. It is not intended to be and should not be interpreted as medical advice or a diagnosis of any health or fitness problem, condition or disease; or a recommendation for a specific test, doctor, care provider, procedure, treatment plan, product, or course of action. Med Help International, Inc. is not a medical or healthcare provider and your use of this Site does not create a doctor / patient relationship. We disclaim all responsibility for the professional qualifications and licensing of, and services provided by, any physician or other health providers posting on or otherwise referred to on this Site and/or any Third Party Site. Never disregard the medical advice of your physician or health professional, or delay in seeking such advice, because of something you read on this Site. We offer this Site AS IS and without any warranties. By using this Site you agree to the following Terms and Conditions. If you think you may have a medical emergency, call your physician or 911 immediately.