How to use "A" an "An" ?

1. The articles a and an is used in certain expressions of quantity with singular countable noun.
2. Use A  before all words that begin with consonant sounds:

• Examples: A door, a pen, a dog, a house, a bus.

     * Some words may be spelled with vowels but be pronounced with consonant sounds:

• Use A when u makes the same sound as the letter y in the word you: a union, a unicorn.
• Use A when o makes the same sound as the letter w in the word won: a one-legged man.
• Do not use A when the h is silent.

     3. Use An  before all words that begin with vowel sounds:

• Examples: An indian, an apple, an eagle, an egg.
• Use An before a silent h: an hour.
• Use An before word that are spelled with consonants but pronounced with vowel sounds: an MBA.

Practice

http://www.learnenglishfeelgood.com/english-indefinite-articles2.html

http://www.englishexercises.org/makeagame/viewgame.asp?id=1203

http://www.eslgamesplus.com/a-an-the-spin/