Get a decent client. Vuze, Azereus etc java-based ones are bloated and don't give you enough control. Transmission gives much more control, and you can e.g. set up 100 down 1 up if that fits you better (or whichever ratio you'll define for your karma).
For ports, check which ones you need open form System Preference's firewall - or if you have a router etc, in it. If in doubt for ports or service, keep it off.