You might even be obligated to join an integration course. This is the case when you have

• a residence permit for at least one year with
• date of issuance no longer than one year ago and
• you do not have a basic level of German (level A1) yet.

When you receive benefits from the Jobcenter (Arbeitslosengeld II), the Jobcenter might oblige you to participate in an integration course.

Exceptions are possible, for example when you already work fulltime or you provide nursing care to a family member at home.

You will receive a certificate of eligibility (Berechtigungsschein) from your immigration authority (Ausländerbehörde). It is valid for one year.

You can then choose where exactly you would like to take the course. Look online for course providers or ask your immigration authority or a Migration Advice Centre (MBE). As soon as you have decided for one provider, you can just go there with your certificate of eligibility and sign in personally.  

When you do NOT participate even though you are obligated to do so, the immigration authority might decide not to renew your residence permit!