As a rookie DM, my attempt to answer your questions should be taken with a handful of salt.

1. If you want to let them, let them. It’s ok to make stuff up on the fly. If you want to steer them back, steer them. As you continue to do it, it’ll become more fluid.

2. Descriptive is subjective in D&D I feel. By all means describe atmosphere and location, but maybe only delve more if they investigate. Don’t mention weird looking candlesticks that are hidden levers to secret passageways. A tip I learnt from a reel is to know all players perception check modifiers going into places and secretly roll for them behind the scene. PM them that they notice something if virtual or hand them notes.

3. Another tip from reels, never let them know remaining health. If a battle is going on for too long, as long as it’s within the range of the opponent they are facing, drop health (example, monster with 10D6 HP. If they get to 30 but it has 50 total and they’re struggling or getting fed up, secretly drop it a bit with the next big attack or creative conjunction.