Back to My Mac will not be available on macOS Mojave. You can get ready now by learning about alternatives for file access, screen sharing, and remote desktop access.
Access files across your devices with iCloud Drive
When you turn on iCloud Drive, your files automatically upload to iCloud, where they're safely stored and available on all of your devices. Turn on the Desktop and Documents Folders option in iCloud Drive to store them in iCloud too. Now you have access to all of your files, on all devices, any time.
You start with 5GB of free iCloud storage and can upgrade any time from any device. iCloud Drive has an individual file-size limit of 50GB.
Access your other Mac with screen sharing
If you have multiple Macs, screen sharing lets you use one Mac to view and control your other Mac remotely. This means you can open, move, and close files and windows, and use apps — even if you're in another location — so you can always get what you need. Learn how to set up and use screen sharing.