The important thing is to think about what you need to show. You could "tell" me that the room is messy. You could "show" me the clothes on the floor, the half-eaten bowl of melted ice-cream, the balls of dust and pet hair in the corners. But if your character is only in the room for ten seconds as they pass through to where they find the dead body that moves the story along, how important is it for me to read all of that? Is this place important? Are you using it to set a particular mood? If not, save the "showing" for things you need me to focus on.
I find a good middle ground is what I call "the unique detail". Here, the key is to focus on what makes this place/person/object different or more noticeable. For example, if you were describing a bedroom, I can usually assume that there's a bed in there and something to keep clothes in. So think about what makes that room your character's bedroom and what the narrator might notice when they walk in. Big white mirror on the wall? Is the narrator a narcissist? Cat curled up on the bed? Is the character an animal lover? Books on the shelf colour-coded and in height order?
This is also an opportunity to use similes or metaphors. Consider these three examples:
1. There was music playing loudly.
2. The stereo was turned up so high that he couldn't hear the words of the song, but it had a steady, pulsating beat that he couldn't help but dance to, nodding his head with each strum of the bass guitar.
3. The throbbing of the bass felt like it was under his skin.
The first is "telling" and not very interesting. The second is "showing" - detailed, with actions that highlight how the music affects the character. The third hones in on a detail (the bass) and a simile (like it was under his skin). It uses fewer words and is perhaps less detailed and evocative than the second, but more effective than the first.
The same principle applies to describing people. Let's move beyond hair and eye colour into what makes that character special. You can choose mannerisms that show us something about the character's personality or past, or just pick something short and sweet that makes them interesting to the reader.
- She had no shoes on.
- He was nearly thirty, but he smelt like baby powder.
- When she spoke, it sounded like her mouth was full of marshmallows.
- He was wearing two wedding rings.
It can be a good exercise to write out the unique detail for places and people you know. Think about what makes your kitchen different from your best friends' kitchen. Consider what would help you to pick out your mum in a crowd.
As a final comment, be aware that "showing" in many cases is about setting a tone or revealing a character's emotion. I'm all in favour of using "show don't tell" as a general rule when it comes to replacing things like "He was angry. He put the glass down." with "He slammed the glass back onto the table, spilling the water."
Keep on writing (and reading)!