After that, you should see a waveform of the MP3 file. Using the selection tool, you can click around the file and use the play button to preview the sound. Once you've found a part of the file you'd like to delete, just highlight it and hit the delete key. If you want to lift a section out of a longer file, you can highlight the selection, use the copy command from the edit menu, and paste the selection into a new file. If you make a mistake, you can always use the undo command to back out of it.