Reaper is a great piece of software for editing audio files. It’s a need-to-pay software, but you can download and use a trial version for 60 days (enough time to complete assignments for most university courses, for instance!). Here are some helpful tutorial videos to get you started with the basics. The videos follow a logical order (building upon information in previous videos). Start with this introduction video, then click the link above for the rest of the series.
A free, open-source audio editing option. Frankly, Audacity is not as good as Reaper, but if you are in a pinch… Download here. There are lots of tutorials, manuals, and wikis on how to use it. Start with these ones.