To say or in Lithuanian, you use ar or arba. The difference between them is that ar has more meanings than arba.

In questions where you have to choose between two things, you use ar:

Jis ar ji? He or she?
Aš ar tu? I or you?
Kas geresni: mes ar jūs? Who are better: we or you?
Kas eis: jie ar jos? Who will go: they or they?

In statements where you don't ask, arba is used:

Jis arba tu. He or you.
Kas eis? Jis arba ji.
Who will go? He or she.
Aš arba Vytautas.
I or Vytautas.

Never use one particle where the other is supposed to be used:

Arba eisi? Ar eisi? (Will you go?)
Eisiu aš ar tu. Eisiu aš arba tu. (Me or you will go.)

You can put ar or arba before options:

Ar aš, ar tu? I or you?
Ar milda, ar Vytautas, ar Laimis, ar tu? Milda or Vytautas or Laimis or you?
Kas geresnis: ar tu, ar Milda? Who is better: you or Milda?
Arba tu, arba jis. Either you or he.
Arba milda, arba Vytautas, arba abu. Milda or Vytautas or both.
Kas geresnis: tu ar Milda? (Who is better: you or Milda?)
Arba aš, arba Milda. (Either me or Milda.)