iMovie is good enough to start with if you're brand new to editing and don't want to spend money.
It's relatively simple and free and you can do all your basic editing functions such as slowing down video and cutting and adding music etc. Since you have a mac you should already have it installed on your computer.
If you want to really get into fun editing functions, Final Cut Pro X is where it's at for Macs. The interface is similar to iMovie except it's a lot more powerful in what you can do with it.
You'll be able to do nice things like advanced color correction, which is very handy when editing GoPro clips that are too bright, washed out or too dark.
It'll set you back a few hundred $$, but it is a very nice piece of software.
It's honestly not even hard to use once you get the hang of it and there's even free video training courses that will teach you the basics in a couple hours (eg - Final Cut Pro X Tutorial, Web-Based and Totally Free! | Izzy Video