The best "authentic" takeaway lunch you can find is at forno (bakery). Normally, here, there’s no room to eat inside, but you can buy freshly baked pizzas and schiacciate (flat breads), and sometimes even fill your sandwich as you fancy. Another alternative to find everything already ready to eat (even in some supermarkets) is the gastronomia (deli), where you can find some pasta and side dishes, desserts, different kinds of meats or custom-make your favourite panino.
If you are a street-food lover you can indulge in a porchetta or beef sandwich at several kiosks located on some city streets. They are well equipped to prepare hot sandwiches and if you are in Florence you can enjoy also some lampredotto or tripe.
If you’d like to eat one or more courses, choose between a pizzeria, a restaurant, a trattoria, a locanda or an osteria. In a pizzeria, in addition to the great pizza (of course) you will often find some cheap yet delicious pasta, risotto or salads. If you want to eat typical local dishes and spend very little regardless of the place or the dish, look for a trattoria.