Study the information about countable and uncountable nouns. Part 1

Anything that can be counted, whether singular - a dog, a house, a friend, etc. or plural - a few books, lots of oranges, etc. is a countable noun.
Anything that cannot be counted is an uncountable noun. Uncountable nouns are not individual objects, they are always singular.
1. Countable nouns can be singular and plural.
A car - cars
New cars are expensive.  

2. With singular countable nouns we use a/an.
I have not got a key.

3. Uncountable have only one form.
Music, money, salt, water
I like rock music.
Use the verb in singular.
4. Don’t use articles a/an with them.

But you can say :

A glass of water
A piece of cheese
A bowl of rice
A can of oil
A  bottle of milk
A cup of coffee
A bar of chocolate
I will buy a bar of chocolate.