They are all prety easy to use. I started on Virtual Box, but I use VMPlayer for virtual machiens at my work.
Normally they have a basic setup where you tell it what kind of OS you will be running and it automatically sets up some defaults.
The program allocates resources from the system you are running but with a OS like Windows 98 it won't take much to run.
After you set up your virtual computer it will be like having a new empty computer with no OS. You can tell the virtual machine to use your CD rom drive and insert a windows 98 disk, or if you have an ISO of windows 98 you can tell the virtual machine to use it instead. If you do not have a copy of Windows 98 just use ReactOS
https://www.reactos.org/download