Can I have on my mac Sierra computer both runing - mac sierra and elementary? How to install elementary on my mac safely - without deleting anything? I want to be able to enter sierra or elementary. Is that possible? Dual boot?!
Yes, of course :)
Just download elementary and make it bootable on USB or CD. Then boot it and click install. Following the instructions, you'll come to step where you can decide if you want to erase whole computer and has only elementary or to istall it alongside other system.
Choose installation alongside other system and you'll see a simple partition manager, where you need to assign some disk space for elementary. Assign as much as you want and complete the installation.
Later when you'll start your computer you'll see grub menu where you can choose which system you want to start. :)
P.S. If you don't see grub menu then you need to enter BIOS and choose default bootloader. But if you'll experiece that problem then write and we'll try to figure it out.