Many of us were taught that no sentence should begin with "but." If that's what you learned, unlearn it - there's no stronger word at the start. It announces a total contrast with what has gone before, and the reader is thereby primed for the change.
William ZinsserWriting organizes and clarifies our thoughts. Writing is how we think our way into a subject and make it our own. Writing enables us to find out what we know-and what we don't know-about whatever we're trying to learn.
William Zinsser