Her comb will get larger and redder right before she begins laying.
She will "squat" submissively when you reach down to pet her.
She may get a little louder right before she begins laying, since she is experiencing new instincts, and she may not be sure exactly what they are telling her to do, yet.
She may go in and out of nest boxes looking for a safe place to lay.
She may try to drive the other hens away from possible nests if she is feeling protective.
After she gets used to the process, things will go more smoothly and she will be less irritable, we promise! (Technically, a female chicken under a year old is called a pullet, by the way!
The Blue Splash Marans pullet pictured below is just about to lay her first eggs. Her comb has just reddened, although it's not full size yet. It will keep growing for another few months after she starts laying.