• Open GarageBand, select “New Project”, then “Empty Project”, and choose the Microphone icon.
• Drag and drop your audio file into the center space.
• To cut out selections, select where you want to cut the track, then split the track. Do this on both ends of the track.
• Split the track by selecting “Edit” then “Split. After splitting the selection, go to “Edit” then “Cut” to remove the selection. Slide the remaining selections back together.
• You can adjust the background noise, to an extent, by turning the equalizer button.
• Once you finish editing your track, go to “Share” then “Export Song to Disk”. Select MP3 when you save.
• Follow the instructions to save to Soundcloud.

You can find additional tutorials and support on Apple’s GarageBand page.