Launch an audio editing program. If you do not have one, download Audacity. It's a freeware audio editing program for both Mac and PC platforms and can perform an array of audio editing techniques.
Click "File", then "Open" and browse for the MP3 you wish to edit. Click "OK" to import the MP3 into the program; it should appear as a visualized wave form in the program window.
Use the selection tool (similar to the tool in word processing programs) to highlight portions of the audio you wish to edit. To delete sections (useful if you want to make an audio clip from the MP3, highlight a section of audio and click "Edit", then "Cut."
Use fade effects to fade in and out of the audio. Highlight a section of the audio you wish to put a fade effect on and click "Effect", then "Fade Out" or "Fade In."
Experiment with the other audio effects that the editing program offers. If ever you do not like the effect added, click "Edit", then "Undo."
Export the edited audio back into MP3 by clicking "File", then "Export." Note: using "File>Save" will not save the file as an MP3 but as the audio program's proprietary file format.