Hello, I am Spanish, so I beg your pardon for my bad English.
The upside down question mark is a must, not an option. You have to put it in the beginning of every question.
Examples:
¿Cómo está usted? --> How are you?
¿Cómo te llamas? --> What's your name?
About the: Hola, ¿cómo estás? <-- This is the right way.
¿Hola, cómo estás? is inconrrect.