I think you are reffering to the lower left wisdom tooth.
Pain can be due to many reasons, food accumulation, gum infection, cavity are a few of them. So until I know the exact reason, it's difficult to give you an accurate remidy.
Anyhow you can start lukewarm salt water rinse thrice daily and medicated mouthwash rinse twice daily for relief.
Consult me on practo for complete medication
Next Steps
Video consult or visit dentist
Health Tips
photograph of the affected area helps in identifying the problem better.