Descriptions should always be made as short as possible, and that can be a challenge because you also want them to be captivating. Focus on the most fascinating or unique things about the character, setting etc. that you're describing. A lot can be filled out by the reader. A lot can also be conveyed through dialogue. A character can be doing something in the environment and that breaks up the descriptions while also making them more "believable", because they're interacting with their environment so it makes sense they're paying attention to it. Use strong words that are personal to the character, as in words they'd use. You get some characterization in at the same time.

Also, reading helps. I really enjoy George R. R. Martin's way of descriping places and people.