How do i delete everything off my hard drive?

ok i want to delete everything off my hard drive becuz i have to much space used and my laptop is moven to slow..it takes like 10 mins to start up and to shut down..its just movin to slow..i wanna delete everything so it can move fast like how when i 1st got it..ok now the only thing is i dnt kno nothin about computers and what i need to do.... do i need to download anything or can i just delete without needing nothing? i have a 2007 COMPAQ windows vista home basics..help any one