Replying to Need to wipe out hard drive on my mac book

my laptop is running out of space on the hard drive so i backed everything up on an external hard drive and i want to delete everything off my computer so i can load everything back up with hopes off deleting any hidden shit and clearing up some space. the computer i have has 60 gb of hard drive and was pretty shitty when my bro gave it to me. how do i erase everything please i need to know how to as soon as possible