"A"? "An"? "The"? nothing? ("The English is the crazy")
Articles ("a/an", "the") are great fun for all the family and ensure that I and other English teachers will never be unemployed. Not, at least, as long as there are 935 million speakers of Mandarin, 144 million Russian speakers, and 40 million speaking Polish - for all of whom (and numerous speakers of other languages) articles are an unfamiliar and confusing concept.
And they have every right to be confused. Why do we say THE Himalayas but not THE Mount Everest? Why do we use the definite article for rivers and canals but not for lakes?
I don't know. But here is a little chart which you can put on your fridge and help you if you're not sure.
There is also a worksheet with some exercises (and answers) at the link below.