How to make an MP3 sound better in Garage Band?

I recorded a 2 min audio thru my iPhone earphones…

then added a soft vocal with punchie bass

it sounds ok but when its played with an audio track it sounds a little noisy /less quality.

IS there anyway to make it more stereo without re-recording with a better MIC?

thanks

Added (1). I also put a blanket over my head when recording; it helped but the final track DOES have SOME abidance from the room still…

I'm not sure I fully understand what you're asking, but I'll answer to what I think you're asking.
The noise heard in the audio is most likely white noise, or some form of background noise. This can be fixed slightly with an EQ, but the only real way is to re-record sadly. If you want the background noise to not be as prevalent then put your mic closer to whatever it is you're recording. As far as making something more stereo, you can't really make a stereo file any more stereo unless you want to get into more in depth and advanced audio editing.
My other answer would be in the conversion of the files, if you're recording directly into Garage band then my first answer is probably the most relevant. If you're taking the file off of your device and putting it onto the computer then there could also be a problem in the conversion of the files. If I'm correct then iPhones record into.m4r files, which can be converted into mp3 or wav files via an online converter. Give that a try and see if that fixes any low quality issues. Sorry that there are no real alternatives…