For many courses you will have to register for the examination. You do this on TUMOnline. On TUMOnline, you can either register for examinations through the overview of your courses, or by searching for the course you want to register to. Two links:
TUMOnline manual (English on page 24)
A quick introduction to using Moodle can be found on https://www.moodle.tum.de/pluginfile.php/791/mod_page/content/14/moodle_short_manual_students_11-05-23.pdf.