I record in my closet these days. I've padded the walls by hanging coats down the walls and putting a blanket over the door; it's a tight fit, but hey, it's pretty noiseproof and there's like no reverb!
You want to minimize background noise and reflective feedback (reverb) when recording. Smaller rooms are generally better and soft walls absorb sound better than hard ones; a quick (if somewhat uncomfortable) solution is to record under a blanket. Background noise is just things like your computer fan, AC units, refridgerators, and so on---avoid having those on or present in the room where you record, if possible.