"Hello and thank you!"
Translation:Γεια σας και ευχαριστώ!
It's correct. You don't have to use σου after γεια. You can even say 'γεια και ευχαριστώ' but bear in mind that this specifically sounds informal and it better not be used when you refer to someone as 'σας'. So, say it to one person or many people in an informal setting, but include at least one 'σας' in formal settings, e.g. 'Γεια σας και ευχαριστώ'. :)
it is a bit complicated because in english they are both referred to as 'you'. 'σου' refers to one person but 'σας' refers to a larger group of people. Hope this helps, feel free to ask anything else, I am greek and I feel like I can give you the correct answer. Also, bear in mind that 'γεαι' is not valid, the correct answer is 'γεια' = ya, meaning hello
It's the same as in German. When talking to one person (informal, friend, family member), you would use the singular "du" ("Du singst ja schön!" or "Wie geht's dir?").
On the contrary, when addressing your speech to many people or to a person with whom you're in rather formal contact (colleagues, client service), you would use the plural "Sie" ("Brauchen Sie etwas?" or "Wie geht's Ihnen?").
Learning takes time and effort, but it's worth it! ;-)
None, pronounce both of them like you would in mass.
Don't fall into the trap of pronouncing ς like sh, that's never the case even if it you think it sounds like it in some words. It's in fact a sound between s and sh, and replacing it with sh sounds really weird to natives' ears. Stick with a simple s and you'll be fine. :)