There are no guest additions for Windows 9x in VirtualBox. More details here on how to set up a virtual machine with that OS:
https://forums.virtualbox.org/viewtopic.php?t=9918
Personally I prefer Microsoft VirtualPC for running Windows 98SE, it had official support for it. MS used to offer VirtualPC 2004 as a free download, don't know if they do anymore. Either way gaming with Windows 98 on a virtual machine can be painful as it lacks hardware 3D acceleration. Many people build a period system to run those games. Even a $20 thrift store special PC can do the task as Pentium IIIs tend to be the sweet spot for Windows 98 use.