Hi, chaves,
It's easy.
With nouns following adjectives, you use either
such a or
so +adjective + a:
Lily is such a charming girl! You have such cute kittens!
It seems to me, the other phrase is not so common in casual speech.
This is not so urgent a matter to discuss it now.
When there is an adjective or an adverb in the final position or when there is no noun, we use
so:
Your new sweater is so cool! She sings so sweetly! Not so fast, young man! It's so wonderful! So wonderful!
Regards
