Please follow the procedure for doing a fresh install of windows 10 - after you select your target hdd it will warn you that all data will be lost. (As it installs win 10 to the target drive it will wipe it clean.):
https://www.microsoft.com/en-au/software-download/windows10startfresh
You say 'hdd' but if you want to invest in an ssd now would be a good time - put the fresh ssd in, leave your hdd disconnected, install to the ssd. (If you do that, don't just reconnect the old hdd afterwards, or you might boot into the old installation and wind up with problems.)
Edit - I made a few changes, it's basically the 'startfresh' procedure above, bearing in mind that you may be wanting to re-activate an existing license as mentioned above.