Avoid using any unregistered audio samples, including movie clips or snippets of other songs in your music. If you are planning on uploading a cover song, you will have to get it registered and approved by the original publisher of the song. This can be a dainty task for someone who isn't "in the business," and can best be accomplished by going through a service such as LimeLight or The Harry Fox Agency. They will charge you for their services; however, they do have access to many publishers and can get you the needed licensing to share your song.
Copyright your music before sharing it with the masses. An original idea is hard to come by, and there are plenty of people out there capable of taking your art and selling it off as their own. Visit the Library of Congress Copyright Office website at http://www.copyright.gov, carefully read over their guidelines and submit your music with the proper forms. As with most anything this isn't free; the basic filing fee is $35 and will go up based upon how and what your are filing.
Render at a high sound quality when creating your MP3 file. Most any recording program that utilizes the MP3 format will give you the option to choose your sound quality. Though it is often stated that 128kbps (kilobits per second) is considered to be CD quality, there is a noticeable difference in the high- and low-end frequencies that causes a muffled sound, particularly on drums at this bit rate. 192kbps is your best bet; most people won't notice any variance in sound quality and you will save on file size.
Choose the online service to which you would like to upload your music. File-sharing programs such as Limewire and Vuze will allow you simply to store your MP3s in a folder on your computer that you wish to share with other users, letting them access your files as long as the program is open and file sharing is enabled.
Online social networking sites like MySpace and Facebook will let you register a band account or page that enables its users to upload a limited amount of music for no charge. Be aware, however, that user agreements for free sites can be invasive (Facebook demands worldwide distribution rights to any material uploaded), and choosing a pay site may be a safer alternative if you plan on selling your music.
Getting your music onto iTunes will warrant the best response to your material, and many online sources such as CD Baby, Reverb Nation and Songcast can make this happen for you. Again, it will cost you some money; shop around for the best deal and choose a service that best suits your financial needs.