Translation:I want to be a police officer.
Lol, who else thought that maybe the speaker wants to do some police officer? Or at least make a new one :D
Good question. 我是 means "I am," of course. 我做 means "I do" or "I make." However, starting a sentence with 做 can mean "being." Using any verb alone can answer a question in the affirmative ("Yes") if that is the verb in the question, but 是 alone is the generic word for "Yes." So 是 is a special verb, kind of like how "be" is unique in English.
Only Scotland, as far as I know: "There's a polis at the door" (https://books.google.nl/books?id=nzm6DwAAQBAJ&pg=PA105). Though it may qualify as Scots rather than English. I have never heard an English or American person call a policeman or policewoman 'a police'.