I have a sore on the inside of my right cheek, near my lips. It is white in color. I've had it for four or five days. What is the best treatment?

Wait it out. If it does not resolve or worsens it should be checked. That being said most of these are blocked minor saliva glands that have become plugged and/or injured such asbiting the cheek. They almost always resolve on do not need treatment.
Topical treatment. There are several over-the -counter products, such as orabase with Benzocaine that can help the symptoms. You might also find it soothing to hold an antacid tablet (such as tums) against the sore to reduce the acidity in the area.