If you don't have enough free space on your computer but you don't really know why, you can get yourself a program that will show you in what files and folders might the problem be. But how to do it?
Solution for Windows
For your Windows, you can use a program called WinDirStat. This program will check each disc in your computer and then show you a detailed overview and a treemap representing individual files and folders. More specifically, every squared area in the treemap is in its size proportional to the size of each file.
Solution for Mac
For your Mac you can download a similar program called Disk Inventory X, that will again show the size of your files in again a very similar graphical layout, as with the case of WinDirStat.
With the help of these two programs, you can easily find some long forgotten files that necessarily take up your free space, and by that effectively clear up the capacity of your computer.
But be careful - you should not delete stuff you don't know what it's for. So it's always better to first backup your data and maybe also ask your potential IT friend about those files or folders you're not sure of.