Hello, It depends on the allergin ie what has caused the allergy,Firstly wash the part thoroughly with clean running water and calendula lotion will help in soothening the the rash. you may take anti histamine tablet like citrizine to start with,if it wont respond then steroids may be needed or you can go for ayurvedic treatment where you have very good medicines to boost the immunity.
Home remedy: 4leaves of basil, tinospora cordifolia stem juice 1tsp along with 1tsp turmeric will be efficient for this.
All the best.