A letter of resignation should generally be addressed and sent to your boss. In large companies, though, it could be the rule that you need to send your letter of resignation to the Human Resources (HR) department as this department is responsible for hiring people in the company.
In the case of resigning from a college or any educational institute, the letter of resignation should be addressed & sent directly to the Principal.
Note, though, that it is generally mandatory to submit a letter of resignation in physical format and it is not sufficient to merely send it via email. It is also best to hand over your resignation letter in person. In the email you may simply mention that you are resigning for "personal reasons" and if your boss/ the Principal specifically asks for reasons you may state them.
To: The Principal
Shri Haridas College
Due to personal reasons, I, Prof. Shirish Das, submit herewith my resignation from the post of Professor, Dept. of Economics from 1st May 2011.
Kindly relive me from my duties and give me the necessary relieving order and a No Objection Certificate.
Prof. Shirish Das