What can cause the labia minora to become very dry and sore? what can you use to help it?

Depends: Your young to get atrophic vaginitis, which is often caused by a lack of or deficiency in hormones that aide in vaginal lubrication/moisture. Certain infections may cause dryness and irritation, so best to get evaluated and see what the best tx would be. In the mean time, use basic lubricants (medicated vaseline.Ky jelly, etc.) during any intercourse to avoid pain and irritation. Best wishes.