A few phrases will carry you through your first day in Madrid:

Spanish	English	When
¡Hola!	Hi! / Hello!	Any time, with anyone
¿Qué tal?	How are you? / How's it going?	Right after hola — casual and very common
¿Cómo estás?	How are you?	To a friend, or anyone you'd call tú
Muy bien, ¿y tú?	Very well, and you?	The standard reply
Encantado / Encantada	Nice to meet you	A man says encantado, a woman encantada

Times of day have their own greetings: Buenos días (good morning — until lunch, which in Spain is often around two), Buenas tardes (good afternoon, and early evening) and Buenas noches (good evening once it's dark, and good night).

Two things to notice: the h is always silent — hola sounds like “OH-la” — and questions and exclamations open with an upside-down mark: ¿…? and ¡…!