Learn Spanish (Latin America): El chico va a la fiesta.

The word va is the "he/she/it" form of the highly irregular verb ir (to go). In Spanish, you usually don't need a separate word for "is" when talking about an action happening right now. El chico va perfectly translates to both "The boy goes" and "The boy is going".

How does the word a in this sentence relate to the question word adónde?

The preposition a means "to". In Spanish, whenever you state a destination, the verb ir (to go) is almost always followed by a. If you wanted to ask where the boy is going, you would use adónde (to where). The question word actually combines a + dónde to ask about the exact destination the boy is going to!

Why is it a la fiesta instead of combining the words into al?

You only combine a + el into the contraction al when the destination is a masculine noun (like el parque). Because fiesta is a feminine noun, it takes the feminine article la. The words a and la never combine, so they must stay separate: a la fiesta.