load the film, crank off two blank frames while making sure the rewind knob is rotating backwards, THEN turn the film counter until it is pointing at one. TAke pictures, rewind film, lather, rinse, repeat.
The din/asa dial is because the Germans, at one time, used the DIN scale instead of ASA which is now ISO. A DIN of 21 is the same as ASA 100, DIN of 27 is the same as 400, and so on (or, in German, 'usw' = 'und so weiter').
Nobody uses DIN any more, but you still find some old meters marked in it.