Do I need a comma...?
I'm trying to write a short story. If I am using two adjectives before a noun, do I need a comma between them?
Example: Entering the attic, I complained about the hot summer air.
Do I need a comma between hot and summer? 
I tried googling it, but I keep getting different answers, so I'm confused.
Comments
You do not need a comma in your example because hot modifies summer, not air.
However, if you were to say something like 'I like the slim, pretty girl', you would need a comma between the two adjectives because slim does not modify pretty. Slim and pretty individually modify girl.
This gets into some of the finer points of grammar and it's good to see that you understand intuitively that there is a distinction.
No comma is necessary because the adjectives are related and pertinent to each other. In other words, they modify each other as well as the noun.
Compare it to, "It was an old, rambling, dilapidated farmhouse." None of the adjectives modify the other. They modify only the noun. Commas are required in this instance.
I would say put a comma after "hot":
"Upon entering the attic, I complained about the hot, summer air."
but it wouldn't be a grievous, grammatical offense if you didn't have one either.
No comma between "hot and summer". But I can't really explain why, and yes, for some phrases you would need a comma.