Here are a few steps for openers:
1.Go to the Tools menu, then dynamic DNS. Sign up for a service. I recommend Dyndns.com. It is free, and will track any changes in your DNS address your cable company makes. You will get a sub-domain from one of their free domains. It would look like this mysite.gotdns.com .
2. Next, make any folders on your computer you want to use sharing, and make sure file and printer sharing is turned on (control panel).
3. Assign a port to use for this service and have it point to your computer. This can be done in the Advanced menu, Virtual Server. The port you use should not be one that can be used by other services. You might try something in the higher range of the available ports, in the 10,000 range.
4.Good luck setting up firewall rules in this router, as you can see by my post below, I'm having a bit of trouble with that myself.
5. To access you computer or any device on your network you simply type in the URL you got from dyndns.com and add a collen after the address with the port number that points to your computer, like this: mysite.gotdns.com:10225.
That will get you started, but I'm going to bet that you have a bit more to do with all the firewall issues I have mentioned before.