The rule of thumb is that you should only delete the files and folders that you can identify as your own, after you make sure you no longer need them - check where the file is located (what is the parent folder, other files in that folder etc.) and what's inside the file (preview the content and file's metadata).
For example, you can delete movies, podcasts, documents, images, unneeded apps, backups etc. Also you can quite safely delete the content of your Downloads folder.
Your question seems to assume that the system could by itself generate a lot of junk, i.e. large deletable files you are not aware of, however that's not the case, as a rule. GGJstudios has given you a good link to where to look for such files in the system, but the truth is that you will only score a couple of GBs by going through the list. It will not save you if your few-hundred-GB disk is almost full.
In reality, the biggest space hogs are usually the files created or downloaded by user, which you should be able to easily identify using a disk visualization tool. Just one ditched season of TV series can free up half of your disk space