A would be used in front of a singular noun that does NOT begin with a vowel. (It indicates one of something).
I see a spider on your shoulder.(I see one spider on your shoulder).
An would be used in front of a singular noun that begins with a vowel. (It also indicates one of something).
I see an elephant at the zoo.(I see one elephant at the zoo).
The plural of both a and an is some.
I see a spider on your shoulder.-----> I see some spiders on your shoulder.
I see an elephant at the zoo.-----> I see some elephants at the zoo.
The can be used in front of a singular or a plural noun It has several usages.
a) The most common would be to indicate a noun or nouns that were previously mentioned.
The spider on your shoulder is red. -----> The spiders on your shoulder are red.
b) Another common usage would be to distinguish one noun or several nouns from a group by using an adjective.
I see the old elephant at the zoo everyday.-----> I see the old elephants at the zoo everyday.
The has other uses as well, but I am out of time.