Countable nouns can be singular or plural:

 a dog  a child  the evening  this party  an umbrella dogs  some children 
 the evenings  these parties  two umbrellas
    
Before singular countable nouns you can use a/an:

 Goodbye! Have a nice evening.
 Do you need an umbrella?
  
You cannot use singular countable nouns alone (without a/the/my etc.):

 She never wears a hat. (not 'She never wears hat')
 Be careful of the dog. (not 'Be careful of dog')
 What a beautiful day!
 I've got a headache.
   
We use a/an ... to say what kind of thing or person something/somebody is:

 A dog is an animal.
 I'm an optimist.
 Tim's father is a doctor.
 Are you a good driver?
 Jill is a really nice person.
 What a lovely dress!
  
We say that somebody has a long nose/a nice face/a strong heart etc.:
Jack has got a long nose. (not 'the long nose')

In sentences like these, we use plural nouns alone (not with 'some'):
Dogs are animals.
Most of my friends are students.
Jill's parents are really nice people.
What awful shoes!
Jack has got blue eyes. (not 'the blue eyes')
 
Remember to use a/an when you say what somebody's job is:

 Sandra is a nurse. (not 'Sandra is nurse')
 Would you like to be an English teacher?
  
You can use some with plural countable nouns. We use some in two ways:
  
i) Some = a number of/a few of/a pair of:
I've seen some good films recently. (not 'I've seen good films')
Some friends of mine are coming to stay at the weekend.
I need some new sunglasses, (= a new pair of sunglasses)

Do not use some when you are talking about things in general
I love bananas. (not 'some bananas')
My aunt is a writer. She writes books. (not 'some books')

Sometimes you can make sentences with or without some (with no difference in meaning):
There are (some) eggs in the fridge if you're hungry.

ii) Some = some but not all
Some children learn very quickly. (but not all children)
Some police officers in Britain carry guns, but most of them don't