Ya has several uses, but its most common meanings are already and now for affirmative sentences, and not anymore for negative sentences.

Ya is usually placed before the verb.

Ya están aquí.
Already they are here.
(They are already here.)

Ya son las once de la mañana.
Already it is eleven in the morning.

Nuestra ropa ya está seca.
Our clothes are already dry.

Ya nada me asombra.
Already nothing surprises me.
(Nothing surprises me anymore.)

The adverbs todavía and aún both mean still or yet. They can be placed before or after the verb.

Todavía no estamos listos.
We're not ready yet.

Mi primo todavía no sabe contar a diez.
My cousin still can't count to ten.

aún vive
he still lives

Aun no cenamos.
Still we haven't eaten.

Aún no estoy convencida con el precio.
I'm not yet convinced about the price.

Note:
Be sure to include the accent of aún.