Countable nouns are usually preceded by an article like the, a, or an.
There’s a chair in the corner.
The report is on my computer.
Uncountable nouns and plural countable nouns are not preceded by the article a or an, but are often preceded by the article the. In many cases, no article is needed.
x There’s a sand in my shoe.
There’s sand in my shoe. (no article needed)
x A doors on this floor are unlocked.
The doors on this floor are unlocked.
The water in the ocean looks brown.
The house has been closed up for a while, so the air is musty.
Many times people will say things like, “I’d like a coffee” or “Let’s get a beer.” In these cases, people are referring to an unspoken yet implied amount that generally would be countable such as a cup, a bottle, or a mug.
I’d like a [cup of] coffee.
Let’s get a [mug] of beer.