Hi, I am Dave, I will help you with this.
1
Generally the way to have a dual boot system is to install Windows first, then install Ubuntu, and that will automatically create the Bootloader using the Ubuntu GRUB Loader.
2
To install Windows 10, you need access to a Windows device, then use the official Media Creation Tool to create a bootable Windows 10 USB on an empty 8GB USB flash drive.
Click this link:
https://www.microsoft.com/en-us/software-downlo...
to download the Media Creation Tool (Click on Download Tool Now), with that you can download the latest Windows 10 ISO (Select Create Installation Media for Another PC), you can create a bootable USB flash drive (min 8GB) using that tool
Then, Boot your PC from the Installation Media you just created (change Boot Order in your BIOS) to begin installing Windows 10
During the installation process skip the steps which ask for a product key and select the option 'I am Reinstalling Windows 10 on this PC'.