A determiner is a word that points to a thing, like a, the, this, my and so on. Determiners say which one. However, since uncountable things can never be "one", so you should know what to do.

>> Learn more about a and the

When either the speaker or the listener do not know "which one" then it is called "indefinite" or unsure. So if I mean any cat, I can say "a cat". But when you do not know or do not care about which uncountable thing (Indefinite uncountable nouns), don't use a or an: 

There is milk in the refrigerator.

There is a milk in the refrigerator.

Remember that a means "one" and uncountable things can never be "one" - they are "some". 

When the speaker or the listener do know "which one" then it is called "definite" or sure. So if I mean the family cat, I can say "the cat" or "our cat". When you do know or do care about which uncountable thing (Indefinite uncountable nouns), use a word that points to a "special one" like this, the, our, my and so on:

Somebody stole money!

Call the police!

Call police!

Some determiners can only be used with uncountable nouns; much and little:

Do you have much jewelry?

Do you have much rings?

I have little jewelry.

I have little rings.

>> Learn more abouut a and the  |  >> Learn more about uncountable nouns