Pretty simple! Sign ¿ goes right at the beginning of the sentence, opening the question, and ? goes at the end, closing it. In correct Spanish, you must open and close question and exclamations ¿Cómo te llamas? ¡Qué calor! Understood? ;) More at www.enforex.com

All question should have both of the question marks. "¿" preceding the sentence, and "?" to finish it. The thing is that a lot of people only use the finishing one like in other languages. It is just a bad and lazy habit, especially extended on internet. e.g. ¿Quieres cenar con nosotros?

¿Qué? X_DDD