If you want to apply for leave, the form “Urlaubsantrag” should be filled out and handed in at the secretary’s office that is responsible for you.
Your leave has to be taken before the end of the calendar year, but a transfer to the following year is possible.
If you work 5 days in a calendar week, you are entitled 30 workdays of leave per calendar year. If your working hours are divided up differently, the amount changes – please check with your supervisor.
Regulations for student workers differ, information can be found here (German only).