'How are you' in Spanish?

My cousins from Mexico are visiting next week.
Somehow they think I know how to speak spanish because my mom said it over the phone. She did it to mess with me.
Now they'll say hi and I don't even know how to say hi back.
How are you supposed to say "how are you" in spanish?

Don't worry, the spanish language is not so hard once you start practicing.
"How are you" in spanish is "como estas" when you say it to just one person. If you're asking it to a group of people you say "como estan".

First impressions count. That's one reason you should learn some of these common greetings.

* Hola — Hello, hi — This greeting is suitable in both formal and informal contexts.
* Hola, aló, diga — Hello (on the telephone) — The choice of telephone greeting varies from location to location. Hola would be understood anywhere but is not customary in many places.
* Adiós — Goodbye — An informal alternative in many areas is chau (sometimes spelled ciao, from Italian).
* ¿Cómo estás? ¿Cómo está? — How are you? — The first form (which is informal) normally would be used with someone you know on a first-name basis or when speaking with a child. The second form generally would be used in other situations. Usage can depend quite a bit on where you are; in some areas, the informal form (estás) would be expected where under the same circumstances the formal form would be used in other areas. If you're a foreigner, chances are no one will criticize you for using the wrong form, although you may be politely corrected.
* Muy bien, gracias — Very well, thank you.
* Buenos días — Good day, good morning — In some areas, a shortened form, buen día, is used.
* Buenas tardes — Good afternoon, good evening — In most areas, buenas tardes should be used in the early evening in preference to buenas noches.
* Buenas noches — Good night — Unlike the English translation, buenas noches can be used as a greeting as well as a farewell.
* ¿Cómo te va? ¿Cómo le va? ¿Qué tal? ¿Qué hay? — How's it going? What's happening? — There is also a variety of colloquial alternatives uses, although many of them depend on the area.
* ¿Qué pasa? — What's happening?
* ¿Qué hubo? ¿Qué onda? — How is it going? What's happening? — These phrases are most common in Mexico.
* ¿Cómo te llamas? ¿Cómo se llama usted? — What's your name? — A literal translation would be "What do you call yourself?" or, somewhat less literally, "What are you called?" The first form normally would be used with a child, or possibly with someone of equal social status at an informal occasion. If you're uncertain which form to use, the second one is safer. Also see the above note. Also see the note with the entry for "Cómo estás?"
* Me llamo (name) — A literal translation would be "I call myself (name)" or, somewhat less literally, "I am called (name)." You can also literally translate the English: Mi nombre es (nombre).
* Mucho gusto. Encantado. — It's a pleasure to meet you. — Either of these could be said upon meeting someone. If you're female, you should say "encantada" instead of "encantado". These literally mean "much pleasure" and "delighted," respectively, so they would have different meanings in other contexts.
* Bienvenido, bienvenida, bienvenidos, bienvenidas — Welcome — Note the difference in number and gender. Bienvenido would be used with a man, bienvenida with a woman, bienvenidas with a group of all females, and bienvenidos with males or a mixed group.

Hope this helps.