Your first responsibility to the reader is to get on with the story. This is exciting and keeps the reader reading your book until the end - something they don't have to do if you bore them with too much description.
Essentially don't sacrifice story for description, as this will turn the reader off really quickly. So keep your descriptions brief, but colorful, to evoke a feeling without being flowery and turning the reader off.
For example, you could describe a character entering an alley at night. You could describe how dirty the alley is by listing all the dirty things: the stink of the bin, the rubbish, the mud, the kitchen smells, the rats, the bums around fires, the flickering lights.
Instead, use this technique for writing brief descriptions: let one or two things SIGNIFY for the whole - e.g. the puddle in this case: "Phillips crept into the dark alley. His foot squelched into a filthy puddle and grey mud oozed over his shoes and soaked into his socks." The reader gets the idea that the alley is filthy, without the author having to make a checklist that bores them. And it is still part of the action of the scene too.
Read some noir detective fiction to help boil down your descriptions to absolute essentials - as this genre is very good at this. e.g. "She's a charming middle age lady with a face like a bucket of mud and if she's washed her hair since Coolidge's second term, I'll eat my spare tire, rim and all" from Farewell, My Lovely by Raymond Chandler. Authors like Dashiel Hammet and Raymond Chandler are recommended, but there are plenty of others.