There is only one way - you need to remove files. If you have files or even programs you never use and do not need, delete them and then be sure to empty the trash to actually free up the HD space they were occupying.
If you have nothing you care to outright delete, you will need to get an external hard drive. Then copy things you do not need on your boot drive to the external and then delete them from the boot drive.
There is no magic to freeing up space. Every file on your drive takes up space - if you are running out, you need to get some of those off the drive to free up space for new files.
P.S. and if you have never actually emptied the trash, the FIRST thing to do is that - files dragged into the trash remain on your hard drive, taking up the same space as they always did, until you actually empty the trash (the difference is being marked for deletion, and actually being deleted).