acquired an 2011 iMac, how do I get it transferred over to me so I can be administrator

Have the person from which you acquired the 2011 iMac follow Apple's procedures outlined in What to do before you sell, give away, or trade in your Mac. Had they done this, you would boot up the Mac as though it were a new unit with all the customary setup steps (e.g. username, password, your Apple ID, etc.). It sounds like they didn't do this and that puts the burden on you to get the full macOS High Sierra installer downloaded to another Mac, create a bootable USB stick, and reinstall the iMac.

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