You should always talk to your pediatrician first to determine the problem. If you have taken the recommended check-ups for the child, the pediatrician may already have recognized your child's problems. He can give you hints for specialists, e.g. Psychologists, psychotherapists who can help your child.
If you are concerned about your child's behavior or have significant difficulties with your child's daily interactions, a family or parenting advice center is the right person to contact. These can usually be found via the youth welfare office of your place of residence.
The health authorities in your city or region also provide advice and support. Health authorities of the federal state in which you live can be found on the website http://www.gesundheitsaemter-deutschland.de/gawebde.htm
An overview of the health offices in Hamburg can be found at: https://paartherapeut-finden.de/gesundheitsaemter/179
Family and Education Counseling Center Hamburg: http://www.hamburg.de/erziehungsberatung/
Youth welfare offices in Hamburg: http://www.hamburgportal.de/hamburg/jugendamt.html
Child and adolescent psychotherapists in Hamburg: https://www.aerztekammer-hamburg.org/arztsuche.html
Parents phone http://www.elterntelefon.de/