Ciamar a tha thu?

Ciamar a tha thu/sibh? How are you?

Tha gu math. I'm well.
Glè mhath. Very good.

Tha gu dòigheil. I'm all right.
Ceart gu leòr. Right enough/not bad.
Chan eil dona. Not bad.

Tha gu dona. I'm bad.
Tha mi tinn. I'm sick.

Tapadh leat/leibh, ciamar a tha thu/sibh fhèin? Thank you, how are you yourself?

thu (you sing.)
sibh (you plural and polite)

leat (with-you sing.)
leibh (with-you plural and polite)

Check out Unit 1 of BBC's Beag air Bheag for sound recordings of some of the expressions:
http://www.bbc.co.uk/alba/foghlam/beag_air_bheag/units/unit_01/

Greetings/Goodbyes

Halò. Hello.
Madainn mhath. Good morning.
Feasgar math. Good afternoon/evening.

Chì mi a-rithist thu/sibh. See you again.
Chì mi a-màireach thu/sibh. See you tomorrow.
Chì mi thu/sibh Diluain. See you Monday.

Tìoraidh. Cheerio.
Mar sin leat. Goodbye.