Ponyboy wants to go to the rumble because he wants revenge for Johnnny and he wants to end the fighting between the Socs and greasers for good.
A “rumble” is a fight between gangs of friends. It is usually a fist fight, but there are sometimes weapons involved.
A rumble, when it's called, is usually born of a grudge fight, and the opponents just happen to bring their friends along. (ch 1, p. 11)
The greasers don’t mind “skin fighting” because it isn’t rough, but they worry about fighting with the Socs because they bring weapons. Ponyboy wants to attend the rumble because it “would settle this Soc-greaser thing once and for all” (ch 7, p. 109). The rumble is supposed to settle something, and end the drama for good.
Ponyboy is still suffering from the trauma of the original fight in the park, running away, the church fire, and Johnny's injuries. He is angry and sad, and just wants to get back at the Socs for what they have done. He also wants the fighting to end.