Where do Johnny and Ponyboy go when they leave the park?
After Johnny and Pony are attacked in the park and Johnny retaliates by killing one of the Socs, the boys knew they would "need money. And maybe a gun. And a plan." They decided to find the one guy who they know they could trust: Dallas Winston. Johnny knew that Dally would be "at Buck Merrill's place. There's a party over there tonight. Dally said something about it this afternoon." Buck was Dally's rodeo partner, and when he answered the door, he wasn't too civil. But when Johnny and Pony demanded to see Dally, Buck "stumbled off" to find him.
I assume that you are asking about what happens in Chaptr 4. In that chapter, Johnny and Pony have gone to the park separately to avoid situations at home. They find each other there are are about to leave when the Socs jump them. Johnny ends up killing Bob Sheldon.
After that, the two of them are kind of panicked about what is going to happen to them. They leave the park to go find Dallas Winston. They think Dally will have a plan to get them out of trouble. He is at the house of a friend, Buck Merrill.