Paladins are hard mode in a city like Skullport. They are expected to follow a couple core tenets:
1. Act with honor
2. Do not willfully commit an evil act or allow your lack of action to let evil continue unheeded
3. Defend and help those who cannot help themselves
4. Help those in need when you are able to at all times
Beyond that, your code is up to you (and you must act with honor to meet any additional vows you've taken). To break a core tenet or a vow is to Fall - to lose paladin progression - and a fallen paladin must atone to take Paladin levels again.
Note that you shouldn't try to stop evil if you are incapable of doing so. Alignment is about impact rather than intent. If you charge into a slave market with your sword drawn and try to attack every guard you see, you will probably be executed, and any future goodly impact you'd have on the world will be completely nullified.