martin.beutler@ulmslabo.de wrote:
> How do you exactly write "tsu" in katakana.?

Search yourself with "katakana stroke order."

For example, this is the site that shows the stroke order
of each katakana with a ShockWave Flash (.swf) movie.
  http://www.nihongoweb.com/Katakanapro/
-- 
Junn Ohta