How to clone iMac hard drive to external drive and clean install OS X

Hello,

My mid 2011 21.5" iMac i5 1TB HDD running Yosemite 10.10.5 has slowed down completely and is not usable for every day applications so I have decided to clone my hard drive and then do a clean install so I can replace all of the wanted data and get rid of what may have caused the problem. The only problem is I am unsure about how to do this. I have recently bought a new Samsung external hard drive which I would like to do this with.

I have booted it in a mode which is usable which I did by holding the alt or option key while turning it on. So I only have access to certain features at the moment such as Disk Utility. I would like to know how to carry out what I want to do so I can get my iMac running smoothly again.

Any help will be great

Thanks