clean wipe

I want to erase everything no my mac yosemite. All encrypted data and all partition drives. Everything. Just like it was brand new. Then i want to install the latest updates (High Sierra) and all the apps . How do I do this? Yosemite 10.10.5 


Model Name: iMac

Model Identifier: iMac15,1

Processor Name: Intel Core i5

Processor Speed: 3.5 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 6 MB

Memory: 8 GB

I was having problems earlier and I called Apple support and he tried to talk me through erasing the HD. I was left with some kind of partition I can't unmount even with the revertible command. How do I start from scratch?

Thanks