Deleted hd iMAC

I bought a iMac of a mate and I thought I’d clean it up as he left all his files etc on it and I’m new to the apple world besides my phone anyways.i went into the section were u delete files and I clicked on hd as all the memory was in there so I did that and re started my Mac.It came up with a circle and a line in it so I read up a bit and they say do option r and it goes into internet recovery but after several minutes later I go into re download the iOS it comes up can’t contact server is there anyway around that??I checked the sever pages and it’s all on line so I’m completely lost thanks.