How much information should you give your players? There's no right answer to this question. It depends on your group, your campaign, and the specific moment in question. So over the next three weeks, we're going to cover a few different scenarios, and my advice on what level of details to share with your party.
Scenario 1: Entering a Room
This is the classic question for new GMs, and one I sometimes struggle with as well. Unless you are using very detailed dungeon tiles or photo reference, you have to find a way to translate the image in your head into the minds of your players. There are some things the party needs to know right away: How big is the room? How many exits? (For some reason, that's the one I always forget to include when describing new environments.) Is there anyone else in the room?
However, you also don't want to overwhelm your party with unnecessary information. Describing in detail the interesting stonework of the walls, the colorful pattern of the carpet, and the way the light flickers off the polished bottles... that might be a bit too much detail for your players.
My solution: Give the party enough information to prompt interesting decisions
Bare minimum, the party needs to know how to interact with the environment. Is it empty, or full of clutter? Are the walls narrow and confining, or wide and cavernous? From there, add in one or two superficial details for the players to latch onto - a smell of mold, the sound of dripping water. Then start laying out the important landmarks - the bookcase of dusty tomes, the throne carved of onyx.
At this point, you can start building a mood into the scene. If you are describing a pub, establish things like: how crowded is the bar? How loud is it? Is there music playing? Are there enough waiters and bar-backs, or do the barmaids look stressed out? If you're describing a fancy ball attended by the nobility, are the people in good spirits? Or is there an undercurrent of quiet murmuring as they discuss their mysterious host? Perhaps they all wear shallow smiles as they try to forget about the impending revolution, but the moment a tray is dropped, half the attendees flinch in fear, expecting assassins at any moment?
Don't be afraid to gloss over the unimportant details; describe everything visible in the room, but be vague about the stuff that doesn't matter. "Around the wizard's study you see walls of books, shelves of strange brass instruments and vials of bubbling liquids." Your players now have enough to fill in the blanks, and you can establish the important landmarks: "Across the room, a large map of the city pinned up on the wall, and framed by dozens of posters of missing children." Now, the players can always go look at the dusty books or the bubbling liquids, but most of them will focus their attention on that map, because it's clearly one of the most interesting things in the room.
Finally, populate the scene. I always establish the layout of a room before I tell them who resides in it. It's not realistic, but it's a necessity. Every time you add a new detail, you are splitting the attention of your audience from what you have just laid out; as a result, any active elements (an out-of-control fire, a clockwork man, a sleeping dragon, a glowing ball of light in the corner) should be established last, as they are most likely to prompt a response from the players. If you tell the party, "You walk through the door, and you see the blood-soaked orc waiting for you," that's all well and good, but now the players won't be listening when you describe the layout of the room - their attention is where it belongs, on the blood-soaked orc.
Next week we'll come back and discuss the second scenario: when your players want to know specific information about how magic items/spells work!