How high or how low should I place the ligature over the reed ?
I know for sure that it has to be placed over the thick part of the reed, but still there is room for moving the ligature up or down. Can you give me some suggestions ? Thanks !

Most mouthpieces have a mark where the ligature should go. If yours does not, place the upper end of the ligature slightly below the point where the filed part becomes bark only.

Ligature placement is also personal preference. The mouthpiece makers often put the lines as guidelines but not always. I like to have my bass clarinet ligature as low as possible. For clarinet, higher.