I have this white sore in my mouth. What is it and what do I do about it?

So you see this in your mouth one day and it hurts a lot.  Especially when you drink orange juice or eat a pizza.  It's usually a small shallow sore inside your mouth (on the gums or inside lips/cheek) that may have redness around the edges.  It's usually caused by stress or trauma and goes away in 1 to 2 weeks.  It's a canker sore or aphthous ulcer.  It's mostly a minor nuisance.  You can use an anesthetic like over the counter Anbesol or use mouthwash like Listerine.  There are other remedies you can use that will require a visit to a medical professional.

"Ulcers with similar clinical features but rarely resolving spontaneously with age may be associated with systemic conditions such as Behçet syndrome, auto-inflammatory syndromes, gastrointestinal disease, or immune defects such as HIV/AIDS. ^{[1]"   http://emedicine.medscape.com/article/867080-overview}