How can you transform a piece written for a piano into a piece for clarinet?

Just play each written note a whole step higher. Also change the key signature, either take away two flats of add two sharps.

more precisely, if the song is in C (no alterations), you need to go one step higher, so in this case to D which is 2 sharps. You just re-write all 1 step higher and then you apply these sharps.

Hey, not so fast!

If you`re doing this in order to be able to play `with` the piano, then of course you have to transpose and play a tone higher.

But if you just want to play it as a solo, you don`t necessarily have to do anything. Just play it as it`s written.

If the piano piece is a song type composition where the right hand plays the melody, then it`s quite straightforward. You simply play that.

However, many pieces will have the melody alternating between both staves, or places where the left hand plays very low notes that aren`t strictly part of the melody but give the piece a certain quality. Then, you have to decide yourself which notes to play to make it sound good.
For example, it may contain a bar where the left hand sudenly plays much lower. This sudden contrast is no doubt important to the whole piece, but impossible to reproduce on the clarinet. Therefore, you could try playing it higher instead.
Hope that helps a little.

Steve