Answer

How do I know which one to use?

That's a very good question. Fortunately, the answer is quite simple. It's about listening to the words you're using. The rule for indefinite article usage is as follows:

Use a before nouns (or adjectives) that start with a consonant sound.

Use an before nouns (or adjectives) that start with a vowel sound.

Here are some examples from our English editing professionals:

Please give the dog a cookie. 
(The noun cookie starts with a consonant sound, so a must be used.)

Please give the dog a delicious cookie. 
(Our editing professionals have put the adjective delicious in front of cookie, but as you can see, delicious still starts with a consonant sound, so a must still used.)

That's an old car. (In this case, the word after the article is old, which starts with a vowel sound. Consequently, our English editors must use an.)

Remember that you're listening here. This isn't about the letter c or d being a consonant or the letter o being a vowel; it's about the sound they make (i.e., vowel sound or consonant sound).Here's another example that might help you understand:

I need to work for an hour before we go to dinner.