I have a red round scaly sore on my arm, is this a sing of ringworm?

Maybe. Ringworm (tinea corporis) presents as a red, scaly, ring plaque with central clearing.
Maybe ringworm. A round scaly sore on the skin can be ringworm, which is caused by a fungus, and the rash is called tinea corporis. Eczema rashes and psoriasis rashes can also cause a round scaly sore and look like ringworm. A primary care doctor or dermatologist (skin doctor) can evaluate to make a diagnosis, and then give details on treatment.