I have covered this before, but it is just so important, I’m going to address it again.
There is a great app available called CleanMyMac. I run it once a month and it gets rid of 10-13 GB of stuff I don’t need. The first time I used it I think it got rid of 70 something GB, but I don’t remember for sure. You can check it out at http://macpaw.com/cleanmymac. A lifetime license is US$29.95 but there are other options.
Sometimes you just have to clean up your own junk which frees up more disk space. Start with your Downloads folder and delete anything that isn’t necessary. Delete large files and archives that are no longer needed. Yes, it takes time, but it can be worth it in the long term.
And while you are at it, uninstall apps you don’t use. This is kinda like your closet. If you haven’t worn it in over a year, you probably aren’t going to wear it again, so get rid of it. Same for software unless it is a very specialized package that you know you only need occasionally, but need to retain. An example – I have a software package that will make counted cross stitch patterns from photographs or drawings. I only need it when someone asks me to make a pattern. On the other hand, if I kept every app I was asked to review but don’t really want for myself, I would have no room on my Mac.
When you delete an app you have to be sure you delete all of it. Microsoft apps in particular like to spew stuff all over your OS and you have no idea where those bits and pieces are. There is an app that will take care of that for you. It is called App Zapper. It cost $12.95. I would buy it if it was $100 because of what it does.
You drag the icon for the app from the Applications folder into the window on the left and it gathers all the bits and pieces and lists them in the window on the right. You click on Zap and the app is gone.