Well, it depends which files you do not need. You can safely erase things in your User folder (things you put there). You did not furnish enough basic information to give spcific help - we'd need your Mac model, and exact OS version.
If you have a lot of videos, music, and apps, we can post links on how to move those permanently to an external drive; here is the latest info which may or may not be the right one for your OS:
Once the libraries are safely on an external drive, you can delete them on your hard drive.
Please note: you need an absolute minimum of 10 - 15 GB of hard drive space available at all times for the OS to operate properly - more is better.