Open the Garageband application on your computer. Select "File," then choose "Open."
Select the Garageband project file containing the song you want to export.
Click the "Share" button, then select "Send Song to iTunes." Choose "MP3" for the file format, and select "Custom" in the the "Audio Settings" section. Select "320 kbps" for the bit rate. Click the "OK" button. The export time will vary widely depending upon the number of tracks you have in the song and the song's total length.
Send your song to a mastering engineer. The last step of recording is called mastering. Mastering helps maximize a song's volume and smooth out song dynamics. Look up a few mastering houses on the internet or ask other musicians who they used for mastering. Every mastering company has different instructions on preparing and submitting music to them. Follow their instructions precisely.
Register with an independent iTunes distributor company. Some of the most popular distribution services include CD Baby, Tunecore and Nimbit. All of these companies charge a small upfront fee to upload your song to iTunes. They also pay you via Paypal for any songs and albums you sell online through the iTunes store. Upload your song to the distributor's website along with artwork you want to include. Be sure to double check all track information to make sure it reads correctly. Most distributor sites like Tunecore and CD Baby take a few days to upload music to the iTunes store.