my lecturer taught me to say, "prendere qualcosa da portare via" (to take/have something to take out/away). There is however another term for this but I forgot. In general they would understand "da portare via". Because I tried that when I was in Italy.
If you want to eat in the restaurant, I would say "prendere or mangiare qua".
Can I help you would most likely be "posso aiutarla?". However some storekeeper would also say "desidera?", meaning would you like to buy something/ desire to buy
Hope that helps! :)