German citizenship for brazilians

Hello.

I am 28 years and born in Brazil. My father is German, but as we didnt really keep in touch, i am only registered under my mothers name.

I was wondering if I could still ask him to register me, so I could become a German Citizen, do you know if its possible?

Thank you.

If your father had German citizenship at the time of your birth, you are German citizen as well.
You need to prove this (i.e. you need official documentation) to the embassy and then you can get a German passport.
The situation with regards to double citizenship is complicated and depends on when you were born - you may or may not have to relinquish other citizenships you have.