Mr/Mrs XXX,
First of all, thank you for offering me this job, and I am very happy to take this job offer.
I am writing this letter to express my pleasure to be a part of your company. I would like to work in the Technical Support Department right now. I do believe that I am capable of this position and would contribute a lot to the company.
I assure you that I will confirm to the rules and regulations of company. And I will do my best to satisfy the requirement of this position.
I'd like to receive the next instruction about my enrollment from you. Thank you all.
Best regards,
xxx
对上文做了些改动，语气比较缓和。