I am very confuse with what the information the websites provided.

In some website on formal writing, they said:

Use active rather than passive voice. e.g. "I broke my promise."

In other website, some say cut down on the usage of ' I, my ... '

? Which to use? Which not to use?

Does it depend on whom the letter is addressed to? I am writing a formal letter to recommend the best idea, and the letter is for the principal of the school. I need to start by introducing myself . What are the best ways to start?

like this? --> e.g. ' I am YCC , the vice-president of the school's asdf Society. I have ... '
( Is this a good start ? But in active voice ? )

You seem to have a good grasp of the situation. Both the advisories you received are sound. Active voice is more direct, and English speakers like directness.

Some people have a habit of beginning too many sentences with "I," which makes their correspondence unpleasant to read.

But, as you say, sometimes YOU are the subject of the correspondence, so you may as well get to the point.

If you read over what you've written, and feel there are too many "I's," you may wish to flip some to passive. "I finished the project on Tuesday," can become, "The project was finished on Tuesday," as long as the information that YOU are the one who did it is either obvious or unimportant.

Your proposed start is fine. Just because you see a "being" verb doesn't mean you're in the passive. "I am the president," is active voice. Sometimes people confuse passive with intransitive. Not all verbs can be transformed to passive voice. "I slept through the meeting," is active, but intransitive. You can't transform that to passive.

Best wishes, - A.
Wow! I AM AMAZED ! You have explained it so well! Thank you so much ! I understand now ! Thank you!!!

I AM GOING TO TRY TO WRITE ONE! Hope you can correct me as much as possible because my english exam is 17 days away Emotion: big smile
Students: Are you brave enough to let our tutors analyse your pronunciation?
pleas help me to change some sentensec in passive to active voice
Emotion: sadplease help me
Of course, but not too many! Emotion: smile - A.
Students: We have free audio pronunciation exercises.

Don't start a letter by saying eg
Dear Mr. Jones,
I am Tom Smith, the vice-president of the school's asdf Society.

In English culture, letter readers are accustomed to looking for your name at the end of the letter.
So, write eg
Dear Mr. Jones,
I am the vice-president of the school's asdf Society.
Yours sincerely,

Tom Smith