The important point is that you have to erase the High Sierra drive (all current data on the drive will be wiped - a backup will be very important) to get from APFS to HFS before you can install Sierra then restore data from your backup. You can not install Sierra on top of High Sierra without erasing the disk first.
The very first thing you should do is download CarbonCopyCloner (free to download and use for 30 days). Get it here: Carbon Copy Cloner - Download
Then, use it to create an HFS+ cloned backup (NOT an APFS copy) of your internal drive as it is now. Of course, you need to have an external drive for this, so, you better get shopping if you don't have one.
Now, you're going to need a way to boot up to Sierra (or an earlier version of the OS) so that you can re-initialize the internal drive and re-install an older version of the OS.
If you DO NOT HAVE ACCESS TO ANOTHER MAC RUNNING SIERRA, perhaps the best way to do this is to go on ebay and buy a USB flash drive with the (low) Sierra installer pre-installed (will run you about $20).
You can use the flash drive to:
1. Erase the internal drive
2. Re-install the OS.
You can -try- using Migration Assistant to "bring over" your old accounts and apps from your cloned backup. This may or may not work (because of the newer OS on the backup).
If that doesn't work, you'll have to mount the backup "in the finder", and start "manually migrating" things back over. This is A LOT OF WORK, but it can be done.
See what you get for not having a cloned backup BEFORE you tried to upgrade to a newer version of the OS?
Hard lesson, but I hope you've learned it.
Go forth from this day forward -- a sadder but wiser young man.
If you DO have access to another Mac running Sierra, use that to download a copy of the Sierra installer, and use it to create the bootable USB flash drive yourself. (I recommend a 16gb flashdrive and one of the following: "Boot Buddy", "DiskMaker X", or "Install Disk Creator")