If the HD is unformatted it should appear in Workbench as a NDOS: icon. If you can see this then just select it and format it.

If however you cannot even see this then the HD hasn't been partitioned yet and you will need to get hold of some Amiga partition software on floppy disk in order to do this.

As for installing Workbench to HD, there are manual and automatic methods for doing this. The simplest way is to just copy all of the contents of the Workbench 3 floppy disk into the root of the HD. The Amiga will then just boot from HD. But to get a full install of Workbench onto the HD you can either manually copy the contents of the whole Workbench 3 disk set onto the HD or use the Workbench 3 install disk that comes as part of the full disk set.