The only food item I've really missed in the US is decent bread. Otherwise, the general food selection in a walmart was close enough to a German supermarket. I guess you'll probably have to resort to ordering regional specialties online unless the region has a sizeable population from that area.
From my experience, people in the US are much more outgoing than they are in Germany - so making friends quickly shouldn't really be a problem. Once you've said something they'll know you grew up in a different place, and college students are usually smart enough to account for that when it comes to little quirks and weird habits. If you're trying your best to be friendly, they'll pick up on it , don't worry. In addition to that, a relative or local friend willing to chaperone you during everyday stuff for the first week or so really helps a ton in my experience. It always helps if there's somebody to patiently explain the little weird things.
Other than that, when I visited my relatives in Texas for the first time, I noticed that people tend to criticize other people more indirectly: "X doesn't look good on you" in German tends to translate to something like "You look great but have you tried wearing x with that instead?". Also, at least from my experience, there is a wider socially acceptable scale of expressed happiness. "OH EM GEE! This is the best gift ever! Thank you, you're awesome. Oh wow, I've always wanted this" tends to be on the same level as "Thank you very much, this is a wonderful present!". People might think you don't appreciate their gift/invitation/whatever if you don't thank them more intensely than would be expected in Germany. It's not really a big problem but it helps to be aware of these differences beforehand. And read a guide on how to deal with the police and a tourist's guide to US law. From what I remember, there tend to be a lot less reglimentations but if you actually break the law you'll usually get punished more harshly than you would for a similar infraction in Germany.
Non-social stuff that seemed weird to me for the first week or so: Taxes are added at the register, so things aren't as cheap as they seem in quite a lot of situations. People considered it acceptable to put plastic covers over their furniture for everyday use.