Hi Andreas,

Do you need a permission, hence a work visa to come to the US and work?
Yes, you do. In order to get one you will have to find an employer first. No sure about the actual procedures. If you are only planning on coming for not more than 3 month you could come on the visa waiver program. You might want to talk to somebody at the US consulate in Germany (either in Berlin or Frankfurt). They might have more info. Or check out their webpage.
If you want more info and if German is easier to communicate
in send me a PM. I will be happy to help if possible.