Yes, you need to register at Arbeitsagentur, but not only at their website, but at the office too. Get your girlfriend to help you. Make sure you got an appointment beforehand.
There are a lot of legal technicalities involved with getting a job as a foreigner in Germany, arbeitsagentur is one of them.
Another is the "Ausländerbehörde", have your girlfriend enquire with them about you and your case before you come to Germany. Get an appointment there too.
All in all, make sure you know everything there is to know about your situation.