If you know what you are talking about you always use "The" at the beginning of the phrase.
If you do you know about the object, or you are not sure, or you are talking about something unknown you use "a" or "an" in singular mode only.
English native speakers usually understand if you use the wrong format. It looks odd.
Please note sometimes you don't use "a" or "an" or "the" at all even if you are going to use a singular word after that. It is something related to the structure of the English language. As you get more familiar with the langauge you can understand them all yourself.