As I see it, the main conflict happens within Tom.
On one side, he has his immature friend Jeff pulling him away from Jessica, telling him how gross she is, and trying to get him interested in the sports car that Jeff's uncle has promised to take the boys out in.
On the other side, Tom feels a shaky sense of obligation toward Jessica, a longing to befriend her that gets stronger as the story continues.
We know that Tom has a lot of conflicting feelings on this issue that he needs to work out within himself. In fact, near the beginning of the story, he doesn't even understand what his mom means when she suggests that he go talk to Jessica. It might help, she tells him. He misinterprets her, because he's still so focused on himself--he thinks she means it'll help him. The internal conflict within Tom is whether or not to reach out to Jessica and find the strength to touch her and talk to her.
This conflict seems to hit its most critical point when Jeff's uncle really does appear with the Cobra, that flashy sports car, and Jeff tells Tom to get in it. But at that moment, he's on his way to see Jessica, who had called and asked him to come over.
It's a serious choice for Tom. Does he make the easy choice--does he jump into that amazing car and have fun with his obnoxious and insensitive friend? Or does he ignore Jeff's exhortations, ignore Jeff calling him a dork, and set off for Jessica's house instead?
It's the latter, of course. That's how the main conflict gets resolved: Tom chooses his growing friendship with Jessica over his poisonous friendship with Jeff.