Transposition of instruments

[Marco Oros]

Hi!
I have one question.
I would like to write something for trumpet and clarinet and I know, 
that this musical instruments are tunned in various pitches, they can't 
use C-key, as for example notation for piano.
So, It means, that I must write c-major song in as-major for clarinet, 
or it is automatically transposed of stave parameter?
Thank You, Marco.i