You can delete anything you've created; it is best not to delete things in system installed libraries or folders. Most of the time, it is Photos, Videos, etc. that are space hoggers, so you can move those to an external hard drive. Once you know they work there, you can delete them off your hard drive. Open a Finder window and go through all the folders under your home folder which includes these.

Highlight a folder and press Command + i keys - you will get a "get info" window which will tell you how many GB's are in that folder.
Please note: your hard drive is more than just storage; the operating system requires at least 10 - 15 GB of free space at all times to perform efficiently. And it reads/writes to the hard drive constantly while you are doing whatever.