This is a good news.
This is good news.
Which one is correct? Clarify the difference
"news" is singular, uncountable. So you can't say "a news," instead you would say "This is good news" just as in "This is good water."
News itself is uncountable. Uncountable nouns don't take indefinite articles a and an; at least not directly. Let me explain.
So far, we understood that
This is *a good news.
is ungrammatical, while the latter form isn't. What if you do need to specify a one about news? That's where classifiers1 come into play.
A classifier, sometimes called a counter word, is a word or morpheme that is used to accompany nouns and can be considered to "classify" the noun depending on the type of its referent. - Wikipedia
The classifier for news, jewelry, meat, cake2 etc. is piece:
Sam just heard this shocking piece of news about the epidemic from the bus radio.
Pfft, this job is a piece of cake for a tough guy like you!
Thus, if you want to make your first sentence grammatical, use piece along with the indefinite article a:
This is a good piece of news.
1: Warning: Do not mix classifiers with noun classes. Inhaling the mixture might result in body inflammation and is highly toxic.
2: Also, a slice of cake.