1. Do I need a title? Is it necessary?
Scholarship letters usually don't require to be titled. If it was not mentioned, I think you don't have to title it
2. Do I start it off with "Dear sir/madam," followed by the 1st paragraph?
You can start with "Dear Selection Committee", or "Dear Scholarship Committee" or simply "To Whom it May Concern". But if you know exactly the name of a person responsible for it, you can start with "Dear Mr. Brown"
3. Do I put the page number on the header?
There are only 2 pages, I don't think you should put numbers on it.
by putting my essay on this site, how do I know that they won't say it is plagiarism?
You can provide your profile information of course, but I don't really think the selection committee will post messages here :)