Here are the basics of 'a', 'an', and 'the':
Using 'a' means that the listener can not guess what you are talking about.
Using 'the' means that the listener can guess or knows what you are talking about.
Of course, there are many exceptions to those rules. I just recently wrote an article about the basics of
a, an, the articles. I use some examples and explanations of the basics. I hope it helps!