cable modem newbie needs help

Ok. After two years using Kppp, I finally have digital cable. I don't know how I got it working. My ISP only supplies instructions for Windows. But for the moment, after some random, uninformed twiddling, I am up. Now, how do I cover by butt? I feel unbelievably vulnerable. What services should I not allow? What should I do about security? I am on SuSE 7.1, KDE 2.2.1. Is the personal firewall useful? Help! Albert Wagner