I am updating out dokuwiki and redid it from zero because in out old version are errors that i cant fix. Now I downloaded the indexmenu plugin and put it in the right folder. How and which documents do I need to modify to add the treestructure to a not yet existing sidebar?
You need to have a page named sidebar (unless the name changed in the config), into the root or the namespace you want, and put among other contents an {{indexmenu}} tag into it. It has a lot of options, so check the documentation.
Note that there is also a rightsidebar for some templates.