There is no FTP server built in to windows 98 like there is later versions of windows.
The only thing I can suggest is that you either learn how to use core FTP server, or find another FTP server that you can use.
for easy to use FTP servers, I recommend bulletproof FTP or ceasarFTP
I think that both of these should work on an old version of windows, and if the latest versions don't you should be able to find a version that does...
If you just want an FTP server, and this isn't so much about you wanting to learn how to use an FTP server program, so much as you just want access to your files outside of your house.
I'd recommend one of these
a little silver external hard drive case called a lan disk, you just put a hard drive in it, and turn on it's FTP server that's built into it.
plug a network cable into your router, (and sort out port forwarding to it's address on the router).
there is an HTTP server on the device (for admin of the device only), so you connect to this to set up FTP user accounts, set permissions of the accounts and check the free space on the disk.
and a samba server so you can just use a regular windows eplorer to get your files when you're at home...
and to be honest, at around £20 including delivery, leaving a computer on will cost more in electricity over a year than these things will cost to buy outright!, assuming that you'd also just take a hard drive out of the computer you were planning to use as an FTP server. else you'd have to buy a hard drive to go in it as well.
(they are about 25W).
(so yeah, basically take less power than a light bulb).