put it up on mediafire.com or soundclick.com. Both are free services for sharing files. Both allow password protection should you need it and are free. Soundclick is designed more for a musician so it'll stream and mediafire is better for file sharing.
I used to be disgusted; now I try to be amused. Elvis Costello
I think the easiest and probably the best way to share an MP3 with an other person is through an email attachment. Done it many times for collabration purposes etc just have to watch the size. If you want to share a piece you've done with the world then sign up as an artist on myspace or soundclick. ark