Grammatik:

"Wia geht's da?" is literally "how goes it (with) you?" or a little bit more freely "how's going?"

A typical bavarian feature is the "'n". It is often added on verbs in a question. It stresses the question and implicates a personal curiosity or is used when the other person does not seem to answer. It is perfectly correct to leave the 'n out. But you will hear it quite often.

Wer bist du?	=	Wer bist'n du?	(Who are you?)
W˚s is des?	=	W˚s is'n des?	(What is this?)
Wia geht's da?	=	Wia geht'sn da?	(How are you?)

"Schee!" is an expression which means that the speaker likes something:

I've painted this picture myselv.	- Schee!
This is my new house.	- Schee!
I've got a girlfriend.	- Schee!
It's sunny today.	- Schee!

"Oh mei!" is used when the speaker regrets something, or feels sympathetic towards others.

I've lost my wallet.	- O mei!
My dog died.	- O mei!
I'm feeling sick.	- O mei!

du/Sie (in-/formal language):

The Bavarian language has a formal and an informal way of addressing other people. Unlike the English, which always uses "you" to another person, there are two word in Bavarian, just as in German. Although in Bavaria the informal one is used far more often.
Informal: du       formal: Sie
Not only the pronound differs, also the verb changes. While speaking formal, you will use the 3.person plural instead of the 2.person singular. But we will look at this later.
In the early stage of this book, you will mainly learn the informal version. This is far more used and it is considered more rude to use the formal way towards a friend, then speaking informal towards a stranger. (Of course when meeting somewho new, Kirk should have spoken formal.)
One exeption is the greeting. So far you have only learned the formal one. But this is rather important because you will need this far more often. When greeting an unknown person you should use this. For example when entering a shop or talking to any person on the street, in an office, etc. Only people who you know the name and who you have talked to before should be greeted informal.

Formal:	Informal:
Griaß God	Griaß de	Hello
Pfiat Eahna God	Pfiat de (God)	Good bye

Here are the formal versions of the phrases you already have learned:

Formal:	Informal:
Wer san Sie?	Wer bist du?	Who are you?
Sie san da Alex.	Du bist da Alex.	You are Alex.
Wia geht's Eahna?	Wia geht's da?	How are you?
Eahna aa?	Dia aa?	You, too?

So far only the pronouns and the verb changes:

du	→   Sie
da	→   Eahna
dia	→   Eahna
bist	→   san

From now on, you will learn both, the formal and the informal words and phrases. But both, the dialogs and the exercises will focus more on informal communication, because you are more likely to need and use this.