¿Como estas?

This is the standard greeting you learn in school. It's a general greeting you can use for formal and informal occasions.

When in doubt you can always use this as the default. However, if this is the only greeting you use, the people you interact with regularly might start to feel you are bit word - or that you aren't progressing in your Spanish.

Now ¿Como estas?, using the second person singular used with tu indicates familiarity and would be considered less formal.

¿Como esta? would be more formal since esta is conjugation for usted that is used in more formal settings.

¿Como te vac?

This means 'How's it going.' You would use this mostly in informal situations. Literally, it means how is it going for you.

The 'it' is missing in the sentence. However, since the missing subject is it, the sentence uses va - the third person singular (he/she/it) of the verb ir (to go).

Notice how it is te not tu. The te references 'you' as an object, not a personal pronoun. We are not saying ¿Como tu vas?, which means how are you going.

¿Que pasa?

This is roughly equivalent to 'What's happening' in English. You would use this mostly in informal settings. This is common greeting among friends.

Literally, it means 'What has passed' or 'What has happened.'

¿Que onda?

This is equivalent to 'What's up.' This is an even more informal greeting used by people who know each other.

Onda literally means 'wave.' However, it's also used to mean 'vibe.' So, in a more literally sense, this question is kind of asking 'What's the vibe?'

¿Que tal?

Is an informal way of asking 'How are you?' It is considered mildly formal.

It would also be considered more formal in Latin American countries than in Spain, where it is a common greeting among friends. Tal usually means such a thing or like that thing.

You can also use Como tal ask about something specific like ¿Qué tal tu día? (How is your day) ¿Qué tal tu familia? (How is your family?)

¿Que cuentas?

Quite literally 'What can you tell me?' This would also be equivalent to 'What's new?'

This is another informal greeting, commonly used when meeting someone you haven't seen in awhile. You would also use this when greeting people your age or younger.

Again, this use the second person singular used with the pronoun tu, indicating a certain familiarity.

Conclusion

So know you know a few different ways to ask people how they are doing when you meeting them.

You can sound a little it more like a native speaker and not drive everyone crazy by always using ¿Como estas?

Hasta luego!