Both Jim and Pap are older men who play, in one way or another, a father role for the young Huck. They are both outcasts from their society, and with little or no means of their own. However, their differences are more stark. First of all, Pap is a no-good drunk who doesn't care a wit what Huck thinks or feels, as long as it isn't making him look bad. Jim, on the other hand, cares about Huck's opinion of him, considers him a friend, treats him nicely, respects his opinion, and looks up to him instead of down on him.
Jim is more of a friend figure for Huck than a father figure. He supports and encourages Huck, and helps to keep him safe. His father just caused him harm and damage, and hurt Huck in numerous ways.
It is also an interesting exercise to look at their similarities and differences from Huck's perspective. He looks down on both of these characters--Pap because he's lazy and drunken, and Jim because he's a slave. He treats both of them with a bit of disdain at times, and pulls tricks and stunts on them. However, by the end of the novel Huck feels that Jim is his equal, and a good friend to boot, whereas he never felt that way about his father.
I hope that those thoughts help to get you started; good luck!