Create the VM
Open Hyper-V Manager
New → Virtual Machine
Give it a name
Assign memory
You can check 'dynamic' for now, and set the range later by right clicking on the VM, choosing settings, and then RAM. For Debian, I use 512-2048, depending on what the VM will be used for, but Debian is fairly lightweight.
Configure networking for your virtual switch
This assumes you've created one, if not, go into virtual switch manager, create one, and assign it to a physical port that has a connection.
set disk space
4gb is usually fine for small applications, again, depends on your use case. this is a bit of a PITA to change later without some boot corruption, so overestimate if anything.
installation options
install an operating system from a bootlable cd/DVD-Rom, image file, path to ISO
right click on your VM, connect