Firstly, set up a workgroup by right-clicking My Computer > Properties > Computer Name and change the default workgroup name to something a more meaningful to your household, also make the computer name recognisable, e.g. MainPC or IBMPC. The computer should reboot. Now identify the folders/files you want to share and right-click them and select share and read or read/write access. A little hand should appear under those that are shared.
Unfortunately Win98SE is a distant memory but it is similar to the above. I think it might be Settings > Control Panel > Network then select Sharing. You should be able to find it.