A or An is used to talk about things which are not specific. These are usually that haven't been mentioned before or that listener is unfamiliar with. let is say i tell i went to see a doctor last week.
explanation i went to see some doctor. i did not mention him before and you are not familiar with him. another option is that it is not important who he is. so i use the word "a"
then i say the doctor said o should get more rest.
now you already know which doctor i am referring to. i am referring to the doctor l went to see. so i use the word "the"
next i say to you do you remember movie we watched together?
i use i tell you well i have seen a better movie since
you are not familiar with the new movie, so i use the word "a"
i also tell you a man on the street stepped on my foot.
you do not know who exactly stepped on my foot, i have never mentioned him before. so i use the word "a"
finally i tell you i am a singer
this sentence simply means that i sing for a living. if i said i am the singer you would understand that i am the singer that was mentioned before.
for example
wow i heard a great singer is coming to town
oh it is me. i am the great singer.