Question: How do I know what details to put in? I want to enrich my story, but not to bury it in a lot of extraneous detail.
Answer: If you haven't read John Gardiner's THE ART OF FICTION, you might want to try it one day. He talks a lot about using specific detail, which means you don't have to invent everything. You can choose details that you already know. I gave a character my son's '66 Austin-Healy the other day and used some real action that took place when some of his friends from college picked up his car and moved it to a different parking lot. When asked why, they said they were looking for a place to stick the key so they could wind it up and just got tired of carrying it around. But the character wasn't my son. She was...well, that's another story. The important thing is that because I could see that little red car clearly, my reader will see it too. I didn't have to "make up" anything.
To this character, movement was essential. She was breezy, bright, and always on a mission. So the car fit for her and for what she was like. If she had been older and a more-serious type, I might have given her a Buick sedan; a soccer mom would get a tan SUV and so on. You choose the details that will add to your character, but you only need one or two and they should tell the reader something about her.
It's important, too, what kind of detail you choose. In a story about a mother's love, a vehicle would be a last choice. Something that she does to show her love for her children would be more appropriate. Also it's important to choose carefully. Beware of choosing a detail with too much portent.
There's an old writing teacher's story about the Russian writer, Anton Checkov, who allegedly told his students when offering advice about creating detail, "If you hang a gun on the wall in the dining room, the story won't be over until someone fires that gun." What this means, essentially, is that a writer must choose small detail carefully. A gun is a heavy portent, it foreshadows action, not family dinner-talk.