I take “Mac 12.1” to mean iMac12,1, 21.5" mid 2011. So if you were able to install macOS 10.13 on the second partition and can boot into it (High Sierra), just run Migration Assistant and select the Sierra partition as the source. It will transfer (i.e. copy) all of your data from the old partition except for the system. I hope you have enough disk space—enough to fit all of the data from the Sierra partition.
If successful and macOS 10.13 is running fine, you should be able to delete the old (Sierra) partition.
If you don’t have enough room, you can get an external drive and run a complete Time Machine backup. Afterwards, you can delete the Sierra partition and migrate your data from the Time Machine drive. Don’t delete the TM drive even after a successful migration because you might need it if you find that High Sierra does not work for you. You can then restore the old partition from Time Machine. In any case, you should have Time Machine permanently.
The iMac12,1 latest macOS supported by Apple is macOS 10.13.6. If you don’t have the latest update installed, you should update first before performing the migration.