Turkish people are perfectionists when it comes to hospitality. They like to think about all the details in advance and make sure you are going to feel like a sultan during the dinner. I'm sure there will be great dishes made for you.
People here usually choose bring sweet stuff with them, when they are invited to a dinner. We even have a saying "Tatli yiyelim, tatli konusalim." literally translates to "Let's eat sweet, talk sweet". You could bring chocolate, dessert, ice cream (we don't eat ice-cream in winter), however it could be a bottle of wine depending on your relationship with the family.
If this is your first time with the family, then I would definitely dress nicely. It is OK to be a little conservative on the first dinner. Don't bring alcohol unless your friend mentioned such a thing. People here may have thick boundaries about drinking alcohol or smoking cigarettes in front of elderly people. But again, I'm talking about the average people in our culture.
I think you should make sure to have a nice hair cut, and showered before you go. Dress nicely, and don't except everything offered. They will be persisting on offering everything on the table. You should not say "yes" to everything. They will probably keep offering you stuff till you give up. Take exactly how much you will be eating, and finish up your food on your plate.
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