The first thing to do is backup your data, although if you were migrating from the PC to the Mac, then you've probably already got an original copy there anyway.
If you have a backup, then probably the easiest thing to do is create a bootable external drive, boot your Mac up from that, and then delete files from the internal drive.
If you have an external USB hard drive, you can start up from macOS Recovery to format it and install macOS. More specifically, you have the following macOS Recovery options:
- commandR to reinstall the latest macOS that was installed on your Mac, without upgrading to a later version
- optioncommandR to upgrade to the latest macOS that is compatible with your Mac
- shiftoptioncommandR to reinstall the macOS that came with your Mac, or the version closest to it that is still available
Broadly speaking, follow these steps:
- Boot up into macOS Recovery using one of the options above
- Use the Disk Utility option to erase/format the external USB drive (NOTE: This means you will lose all data on it. Another option is to use a USB Thumb Drive of at least 8GB in size, although this will probably be a lot slower).
- Once it's formatted, install macOS. (This will take a while).
- Once complete, boot up from the external drive by holding the option key during startup and then selecting it to boot off.
- Once booted up into macOS from your external drive, your Mac's internal drive should be accessible as if it's an external drive.
- Delete files from the internal drive. (Note: You will want to ensure you have at least about 9GB of free space in order for macOS to function properly. However, my recommendation is that you generally was to have no less than about 15% of your drive's capacity free).
- Make sure you don't forget to empty the Trash (Finder > Empty Trash...)
- When you've freed up enough space, go to Apple > System Preferences > Startup Disk and make sure you've selected the internal drive as your startup disk
- Shut Down your Mac (Apple > Shut Down...)
Barring any other issues, you should now be able to boot up from your internal drive.