How do I know how much detail/description to use in a story? How do I know when it’s too much?

You want to use the amount of detail and description that will make the the scene feel real and immediate, but not so much that the reader feels overloaded with unecessary details. The key is to use details that are relevant to what you’re describing, and that matter to the story, and that are not themselves already obvious or self-evident (there is no need to make a note of the sky being blue, unless the fact that is is blue is interesting or relevant or unexpected in some way.) Description should also not be dropped in the middle of, say, an action scene, as it slows down the action. Here’s a good article from Bella Online about describing characters. Beth Hill also has some good guidelines for how to use description.