For most adjectives, the endings are like this:
o : masculine singular
os : masculine plural
a : feminine singular
as : feminine plural
La abuela means the grandmother. It is a feminine singular word. Therefore, you use the ending -a, so the answer is vieja.
Also, keep in mind that the adjective endings match noun endings. Abuela has the ending -a, so the adjective should also end in -a.
I believe it's Voy, because that is the "I" or "Yo" form of the word "ir", meaning "to go".
Where are you going to eat?
(I am going to) The restaurant.
¿Adónde vas para comer?
Voy al restaurante.
