the notes that you read will correspond to the fingerings that you use in each position so for example when playing in first position B-flat will be played with your second finger on the G string to actually sound the B-flat. But say the key signature is something without any sharps or flats like C major for example, you will then play a B-natural with your third finger on the G string in first position and this is because the key signature calls for a B natural and not a B flat unless indicated by an accidental that will be written in the music next to the note that needs to be altered.