As an employee, you are entitled to self-declared sick leave, i.e. to notify your employer that you are unable to work due to illness without having to present a medical certificate. The main rule is that self-declared sick leave can be used for up to three calendar days at a time.
For more than three calendar days you must present a medical certificate from a doctor. Self-declared sick leave can be used four times in the course of a 12-month period.
You must have been employed for at least two months to be entitled to take self-certified sick leave. The conditions for self-certified sick leave may differ. Ask your employer about the rules in your workplace.