In the center of Spain we say Hola, ¿qué tal? or ¿qué hay?. We could say in more informal way ¿qué pasa? or ¿qué pasa tío?. Other way, very used in Andalucía, is ¿qué pasa pisha? that means "What's up dick?" but it is used like "What's up men?" This way is only used in Andalucía and in my opinion it donesn't sound well by foreing people, don't use it. Tío means "uncle" but it is used in Spain like "guy". I think, the best way is: Hola ¿Qué tal? but this is very personal. For me (27 years old living in Madrid), a common conversation start like this: Me -Hola qué tal. My friend - Albano (my name) ¿cómo vas? Me . - Bien, ¿y tú?

You can also say: "Hola, como estas?" or "Que gusto verte!"

To start with, you can say simply 'Hola' or 'Buenos días/Buenas tardes/Buenas noches' to unknown people. I guess you already knew these. Once you know the person, a plain greeting could be 'Qué hay' or 'Qué pasa' (something like 'what's up?'). Many young people simply say '¡Eeeeeyyyy!' when they meet at bars or the like, you know, often shaking hands. Of course, you can still use 'Hola', it doesn't sound too formal at all (pronunciation is not so strict, so can enlarge vowels at will: 'Hoolaaaaaaaa'). If you haven't seen the person for a long time, you can ask something like '¿Cómo te va?' (something like 'how have you been?'). Also, when speaking to friends, many people add words like 'tío' ('dude') or 'colega' ('mate'). When people are too happy to see their friends (and if there is real confidence), sometimes they use slang words like 'Hijoputa' (literally, 'son of a bitch'). This can be confusing, and usually not recommended, but if you listen to Spanish people talking between them, surely you'll hear this. Rules to apply this kind of vocabulary are complex, so better don't use it if you are not sure with recently known people. Examples: '¡Qué pasaaa, tío! ¿Qué hay?' - 'What's up, duuuude! What's the matter?' 'Hoooolaaaa, qué tal todoo?' - 'Heloooo! How's everything?' 'Eeeeeeey, colegaaa' - 'Eeeeeeeey, duude' (surelly you'll hear this, but be careful): '¡Hiiijopuuuta, cuánto tiempo!' - 'Damn it! So much time!'

I live in the U.S. very close to the Mexico border, here we say stuff like Que honda way? Como andas? Que paso vato? You say these when you are greeting friends, hope it helps

Hola que más? or que tal? encantado/a de conocerte, cómo va todo?