For all the tasks you mention above, any version of Windows server will accomplish this. The Windows standard server will allow you to...
"do file sharing, have a backup of his files, share printer and able to work from home."
The standard version of Win 2000, Win 2003, even Win NT server will do all that. Consider the business server if you wish to have all the additional services provided.
To work from home, you will need to make a connection to your network which can be a modem on your server attached to a standard phone line or through the internet. Use routing and remote access to configure. With the size you give, I'd do a phone line connection over a modem and then make a remote desktop connection to the server as well(basically like being physically logged into the server from home). The remote desktop gives good response over a modem connection. I would finish your education before attempting/upgrading to a connection through the internet, as the security risk involved requires more than basic knowledge.
A+/MCP/MCSE/MCDBA