Open iTunes. Click "Edit" on the top toolbar, then select the "General" tab. Select "Import Settings" and click the "MP3 Encoder" option and click "OK." For music files already imported, right-click on the desired song, choose "Create MP3 Version" and iTunes will create a new, MP3 file under the old one. For CDs, insert the desired disc, click "Yes" on the pop-up window asking if you want the CD to be imported, and iTunes will save the files as MP3 songs.
Your Windows Media Player also has settings where new music file will be automatically converted into an MP3. Open your Windows Media Player and click "Tools" on the top bar, then hit the "Options" on the pull-down menu. Click the "Rip Music" tab and choose "MP3" under the Rip settings. Hit "Apply," and your new settings are done. Insert the desired CD, Click rip and let the Media Player save your music as an mP3 file.