Scout, listening to the warnings from Atticus about leaving Boo Radley alone, is trying to do just that when Dill and Jem hatch the "let's look into his window" scheme. She also has realized that Boo is probably often watching all of them in their different shenanigans; the day she fell into his yard out of the tire she heard someone laughing. So, she knows that they are probably being watched by Boo, and that Atticus has asked them to leave the poor man alone.
So, Scout protests Dill and Jem's plans to peek in the window. She is scared, she is whining, and she won't stop pestering them. The first thing Dill and Jem try is getting her to go home. She is immediately suspicious. The second thing they try is telling Scout to "keep her trap shut," and then finally, Jem declares, "I declare to the Lord you're gettin' more like a girl every day!" That does it. Scout hates being called a girl; she hates everything girlie, and can't pass up a challenge. Her reaction to that threat and challenge is "With that, I had no option but to join them." All of this happens a couple pages into chapter six, if you're looking for the quotes, and they go to show just what a sucker for a dare that Scout really is, especially one threatening her tomboyish nature. I hope that helped; good luck!