I'm new to writing, as I'm currently working on the first draft of my short story. However, I was wondering when to keep descriptions brief and to the point and when to describe things in depth, using metaphors and everything. For example, if a character is stepping outside of their house, should I just mention it in one line, or should I describe them twisting the doorknob, pushing the door open, etc? Also, how do I know that I am not over describing?
This one can be super tough, honestly. I like to sort of treat it like shots in a film -- what will convey close shots, slow zooms, or sweeping pans? Know where and what your "camera" is looking at. Will a quick swing over the set be more efficient for laying the land, or are the details too crucial to gloss over? The specific elements that you prioritize are going to depend on you, but I'd say there are two big ways to filter information:
Information. What is important to the reader's understanding of the scene? Does it enrich the description to know exactly how the character turns the doorknob? For instance, if Daniel's drunk enough to fumble over his keys at the door, this tells us something about the state he's in, so it serves a purpose to the scene. If he's just opening the door to head out for work, though, it would be a little weird and distracting to walk through an action as simple and thoughtless as twisting the knob and pushing the door open.
Characterization. What is important to the character's understanding of the scene? What does your character value? If Daniel's a rich city-goer visiting the countryside, he might look disdainfully on the dirty clothes or rough hands of farm workers, something a local would likely never notice or comment on, and therefore never describe. Conversely, one of those farmhands might be utterly stunned at the sheer scale of skyscrapers that Daniel takes for granted and wouldn't think to mention.
Either way, make sure the details you include have a purpose -- either to paint a better sensory picture for your reader, or to inform them about the perspective character(s) and what they value.
What does your point of view character notice? Put in details that they would notice and omit those that they would not. Consider what details catch your own attention when you open a door. The space on the other side, the people, the smell, the temperature, the humidity, differences in air pressure, the list is nearly endless.
Certainly including as much as you could imagine would likely get tedious and boring, and likely repetitive, to read so that sets the upper limit on how much detail to include.
Notice how your own mood affects what you notice when walking into a new space, smells of food if you're hungry, the presence or absence of a friend when you are lonely, and so on. By including the details that a sympathetic or heightened by your character's internal emotional state you can make the space more real to the reader and make your character more relatable, with an economy of words.
tl;dr; Include the details that move the story forward and expose and share character internal state and are needed to build suspense or tension.
I would recommend keeping it shorter for most things. Of course, if you are writing a mystery novel, then little details can be more important if they solve the case. There are also some points in the story where a lot of detail is better. I'm about to post chp2 of one of my stories, and although I don't do a ton of detail everywhere, I have a scene where every little thing is described.
These little places where everything is described can really emphasize a point, I like to use them at character deaths, introducing new characters, love at first sight(I'm not proud of it but it works -_-), and when there's just a big change in scenery. I wouldn't do this right in the middle of a fight scene or something because it can easily interrupt the flow, but if you do it right it could work. I still wouldn't recommend trying it until you're a bit better at writing though.
For your second question; if you spend more time doing things than moving the plot or talking between the MCs, then you definitely need to dial it back some. You might have a hard time deciding when is too much, so for that I would recommend finding beta readers(they pretty much proof read everything) to catch your mistakes for you. You might feel a little dumb when they send back the document full of red ink, but they're still helpful and help a ton. Here's a link to a beta chat room, but you will need 20 reputation to chat.