Information begets information and eventually, there's just too much of it. I don't really believe that -- I'm an information junkie -- but there's no question that if you never delete anything, eventually, you will either use up all your disk space, or you'll have a terrible mess on your hands. Deleting and renaming files and directories is usually pretty simple stuff, but things can get weird and sometimes, simple is dangerous. Let's start with the basics.
To delete a file, use the rm command.