Huck Finn, in the opening chapter, has been taken in by Miss Watson, and is having a hard time feeling happy and comfortable within the confines of a civilized and routine life. In fact, he's so restless about it, Huck, after being taught about Hell, proclaims he wants to go there, because "all I wanted was to go somewheres; I warn't particular." So, he is already of the mindset that this life was too boring and restrictive for him. Then, at the end of the chapter, there is the incident with the spider. Huck flicks a spider into the candle flame, and his response is,
"I didn't need anybody to tell me that that was an awful bad sign and would fetch me some bad luck, so I was scared and most shook the clothes off of me."
So, Twain introduces foreshadowing here, through the sometimes bizarre superstitions that Huck and people of his time and place hold. Already, there is an ominous tone foretelling his Pap's return, which leads to the escape. This happens again in a later chapter when Huck spills salt and Miss Watson "crossed [him] off" from tossing some over his shoulder, which is another sign of bad luck.
Huck suspects that bad luck was accurate when he finds the boot print in the snow, and very wisely "sells" all of his money to the Judge so Pap can't get a hold of it, just in case he's hanging around. When Pap does show up, and kidnaps Huck, Twain is in full swing for setting up the escape by then.
Twain also introduces Jim right off the bat as Tom and Huck play the prank on him regarding the hat. Introducing Jim right away, so that we get to know him a bit in a normal way, sets us up to be familiar with him when Huck runs into him on the island. Later, Huck even goes to Jim for the "hairball" fortune, so we get another encounter with him. When they meet on the island, they are two familiar characters that we are ready to set off on a journey with.
I hope that those thoughts help a bit; good luck!