When I am trying to select and delete one part of an audio file, it always selects and deletes the other files I imported. Please help me. 
If you’re unsure about “selecting” the section of audio to be deleted, see: http://manual.audacityteam.org/man/audacity_selection.html
If you are selecting the audio correctly, then my guess is that you have accidentally enabled “Sync-Lock”. See here for how to turn Sync-Lock on and off (it should normally be off): http://manual.audacityteam.org/man/sync_locked_track_groups.html