A tapa is a small portion of food served on a small dish or clay pot. It may be cold or hot, and it can be shared or eaten individually.
A ración is portion of food more abundant than the tapa. It is generally served on a larger dish, and it is meant to be shared by a group of people. You may order about 2 or 3 raciones for a group formed by 4 people. You can also order a media ración (half portion), which may be perfect if you want to taste a wider range of specialties!
You must take into account that if you want to order a bottle of beer, you can say una cerveza. If you prefer draft beer, you can order una caña (smaller than half a pint) or una pinta (a pint). In addition, you can order una clara (a glass of beer shandy).
If you want to order wine, just say una copa de vino (a glass of wine), and add tinto or blanco so as to refer to red or white wine, correspondingly.