a/an

If a word that follows starts with a vowel sound you need "an".

Please hand me an apple.

You're going to need an X-ray.

Although "x-ray starts with an X, it starts with a vowel sound, it sounds like an "e" as in "every".

If the word that follows starts with a consonant sound, you need "a".

I need a pencil.

This is a very important lesson.

More examples:

He has a brother and an older sister.

We got into an argument yesterday.

He' an honorable man.