Get the right address:
The first and foremost step is that you should get the address of the person to whom you'll be sending the letter. You should make sure that the person you're sending your letter to is the concerned person to deal with your application. Several universities have different departments or designated personnel to deal with the applications classified upon their nature of program. For example, if you're interested in seeking admission in the electrical engineering degree then make sure that you addressed your application to the concerned department instead of just engineering, or any other department.
The first paragraph of your letter should clearly state your intent. This section should explain what's the purpose of the letter, what department you want to enrol in and what semester you're seeking to admit in. Once you're done expressing your intent, proceed with describing your educational background, experiences and other such details that might help your case. The paragraph should serve as a summary of the whole letter, being brief and to the point.
The second paragraph should include the details of the experience that you have gathered during your course till your current position. This can include your work experiences, volunteer work and other such activities that might be deemed as relative to the field you're applying for. Keep that in mind that you need to only highlight the relevant experience. You can also state other experiences as well, however, that wouldn't do you much good.
The details regarding experience should be followed by a more casual paragraph where you explain that why you want to enrol yourself in that particular course and what are your future plans after completing that degree.
At the end of your letter don't forget to state your commitment and determination for the achievement of your goals. Moreover you should also state that you would do your best to excel in the program, while upholding the ethical values of the institution.
The 3 Objectives:
The objective of the statement is to accomplish three things. These include that it should achieve a solid demonstration of your interest in the program, knowledge of what the program has to offer, and how your preparation and experience make you a good match for the program.