First of all you need to be running some sort of file-sharing service on your office computer. If its a PC, that'll probably be Windows file sharing. If its a Mac, it'll most likely be AFP.
Next, your work computer needs to be accessible for that protocol from the outside world. If you're on a company network, you'll need to speak to your network admin to find out if they can set this up for you. This usually involves "port forwarding", which means connecting to a given port on the company's address will take you to your computer. Your network admin will know what to do.
On the other hand, if you're connected directly to the Internet and have your own IP address (unlikely, but it does happen) then you can skip that whole step and just use that IP address to connect from home.
The devil is in the details, kbkim, so we'll need to know what your situation is.