Dear (Name of HR)
It is my great pleasure to inform you that I will be accepting the offer of employment of (the job title and company name). The goal of this position is in line with my career aspirations, and I am hoping to grow and learn in this position.
As discussed in the interview, my salary would be (salary amount), and I will start on the (starting date and location).
I appreciate the time you took to facilitate a seamless interview process, and I am looking forward to working with you.