Music in MX

Hi, I don’t know too much about music for websites, however I’ve had some luck using sound loops from sites like I have a web client who would like some of her original music as background music on her website,, but the clip she sent me is too big. (13MB!) What is the best file format for the web and how does one go about making a web music file from an MP3 or something?? obviously, I have no music mixing or recording background! Help!