i only use my computer for surfing the net but space still diminishes even if i clear all history

In addition to clearing your browser history you will need to empty the cache by selecting it from the Safari menu and removing the cookies from the Preferences Menu. You can hold down option, command, and E to empty the cache.
Do you have an iTunes Music Library, use Mail, or talk using an instant messenger? That can also take up more space on the hard drive.