I have an itchy red patch on the inside of my elbow. It has been there a month. What is this, and what should I do?

Skin patch on arm. This may be a form of psoriasis or eczema. Look at some pictures online to compare and schedule a consultation with dermatotolgist for evaluation.
Atopic dermatitis. Atopic dermatitis is commonly found in flexural surfaces such as the inner sides of the elbows. However, it needs to be fully evaluated and examined to know what it is. Please see a physician. In the meantime, you could try otc Hydrocortisone cream.