Within each Master track, some courses are obliged or strongly recommended. Others may be selected freely. Discuss the possibilities with the coordinator of your Master track. Ultimately you will have to compose a master programme which contains at least 120 EC within a month after starting the master programme.
In case you are not sure about the choice of your (elective) courses still fill in programme registration form which can be found on the blackboard organization of your track. You can change your study programme at any time but always in consultation with your track coordinator by restarting the process.