unmounted and deleted Macintosh HD, now i cant reinstall OS El Capitan

I got a brand new iMac, 1 tb edition about a month ago. a week ago the computer started acting really weird. everything was super slow, color wheel all the time (it froze up in finder for gosh sakes). basically it was completely unusable, firefox took 5 minutes to open. so i decided to wipe the computer and start over. so, i used the directions given by macworld.co.uk and they instruct me to unmount my hard drive and then delete it. apparently this is a big no no (thanks macworld). ive spent all day working with apple support and we cant seem to get it to work. i think i have to create a bootable installer on a flash drive, but im not quite sure how to do that, the apple support instructions are really vague and i dont want to listen to macworld again. Any tips or suggestions?