Using the articles "a" or "an" is often tricky because there are no exact rules on usage. It mainly depends on how the word is pronounced and how the first letter of a word sounds when deciding which one to use. However, here are a couple tips to help decipher which is more appropriate in a sentence.
Step 1
Use the "a" article before all words that begin with consonants. 
For example, "a cat" or "a donkey."
Step 2
Use the "an" article before any silent h. This is an exception to the above rule. For example, "an honor."
Step 3
Put "an" before words that begin with vowels. For example, "an apple" or "an enemy."
Step 4
Consider the two exceptions to Step 3. When "u" sounds like "y" in "you", or "o" sounds like "w" in "one", then use "a". For example, "a user friendly device" or "a one man show."
Step 5
Remember which article to use by basing it on the sound of the first letter in a word, not the actual letter. If the first letter sounds like a vowel, you use "an." If the first letter sounds like a consonant, use "a."
Step 6
Try not to confuse which article to use when you are dealing with acronyms and other abbreviations. For example, it is correct to say "an MRI", even though it begins with a consonant. This is because "an" can only go before vowels, and you pronounce MRI "em are eye." It sounds as if it begins with a vowel.