hello - labas
goodbye - viso gero
how are you? -
formal (single or plural)/familiar plural: kaip laikotės?
familiar single: kaip laikaisi?
what's up - this is a slang saying, and therefore it doesn't translate, but since it means more or less the same as how are you, you can use the translation for how are you.
there is one natoinal character (the e in "kaip laikotės" should have a dot over it), so please view through Baltic encoding/character set

hello - labas, sveikas (or: sveika-feminine, sveiki-plural masculine, sveikos - plural feminine)
goodbye - viso gero (or "viso" - familiar)
how are you? - "Kaip laikotės?" (plural)
"Kaip laikaisi?" (familiar single)
"Kaip reikalai?", "Kaip gyvenimas?"
what's up - Kas atsitiko?