Como estas is used for single people and also addressing a group correct? i.e. "How are all of you?"

I normally say in a restaurant "Cuales cervezas tienes?" but recently someone corrected me to say "Cuales cervezas tienen" but I can't understand why use "tienen"?

How do you say "I would like...", I could say dame (give me) or "yo quiero" (I want) but that sounds rude? In French you'd say "Je voudrais.." instead of "Je veux" which is considered rude. Is there something similar in Spanish?

Also how would you say "Would you get me the bill"

Thanks!

Como estás is used for single people, for groups you use "como están" (if you're talking to people you refer to with Usted rather than tú or if you're in Mexico where they only use the ustedes form for the plural, no metter if they use usted or tu in singular form) or "cómo estais" if you're in Spain or talk to people with whom you use "tu".
Cuales cervezas tienes? ... same problem, you're referring to one person using the tú form (not formal), in Mexico you'd rather not use it if you do not know people well and "cuales cervezas tienen (ustedes)" means that you're referring to more than one person and using the usted/ustedes form (formal).
Dame sounds rude in Mexico (and half rude even in Spain unless you're talking to a friend or relative), you can use "Quisiera" (Je voudrais, I would like).
Would you get me the bill... in Spain you can say "la cuenta por favor", my mexican friends use "me podría regalar la cuenta por favor?" (lit. could you give me the bill (as a present)?) We have great fun in Spain seeing the faces of the Spanish waiters when my friends use this expression!!!! I think that the "Spanish" version could sound polite enough even in Mexico...

Muchas Gracias Giulia!!!

I had one more quick question...I've noticed some people pronounce "yo" like "jo" and also "llegar" as "jegar", i.e. the double L with the J sound instead of a Y sound. Is this a South American thing?

De nada!
I suppose that when you write "j" you mean a French sounding "j"... if I'm right it's mostly an Argentinian way of pronouncing the Y/LL sound that is one of the most controversial sounds in Spanish...