Instead of writing a flat sign before every Bb note, it is easier to write just one flat sign after the treble clef. This means that all B notes on the staff are played as Bb , even though there is no flat sign placed before the note. This is called a Key Signature. For more information, see the section on Key Signatures in Lesson 9.
Play all B notes as Bb as indicated by the key signature.
Sometimes it takes only two or three notes to create a good saxophone part for a song. In a song like this, where and how the notes are played is the most important thing.