Got early accepted into grad school: happy, nervous, concerned [on hold]

I am still trying to piece together everything that happened over the course of 3 hours but the story went like this:

Prof A contact me said he saw my application and was impressed with my academia drive and motivation and wanted an interview. I contacted him and said I was available for an interview right now. I went in for an interview, which is really a chat about my academic background and my interest. Now he has sent me an offer to join his research group.

Of course I was elated when first got the offer. Now I am a bit concerned and curious and my mind is filled with questions "like why did he pick me?" "I was never the best student" "I must be a fodder to fund more talented graduate students" "Will they use me for some menial tasks that requires 60 hours a week of data analysis?"...

The more I think the more questions I have. I am also uncertain as to what type of research he would like for me to do. He showed me some publications and all of them are at too high of my current skill set. Then he asked for my interest and my response was along the line of...anything really.

Has anyone ever been in the same situation before? You have been accepted as a graduate student while you are still finishing up your undergrad, what are some of the sequence of steps that happens afterwards? Does anyone know how your direction of research is decided and how can I sway the prof to a direction that I wish to conduct my research in?