When a pullet will begin to lay varies from breed to breed, with production breeds starting as early as 16 weeks, other breeds sometimes as late as 6 months or more. It will also depend on when they were hatched, young birds that were hatched in the late summer may not lay until the following spring.
Signs that your young pullet is getting ready to come into lay include the fact that her comb will get larger and redder, her face will get red rather than pink, and she will begin to "squat" submissively when you reach down to pick her up.
As well, a hen or pullet who is in lay will have a moist, pink vent, and a wide, plump pelvis. With large fowl, check the space above her vent on her abdomen, feeling with your hand until you find the two pelvic bones. On a large hen, 3 fingers should be able to fit between those bones on a hen that is currently laying. If you can only fit 2 fingers, the hen is not laying. On a bantam, you have to consider the size of the hen and egg.
Additionally, she may get a little louder right before she begins, often squawking loudly in or around the nest boxes, 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, and she may try to drive the other hens away from possible egg depositories if she is feeling protective. After she gets used to the process, things will go more smoothly and she will be less irritable.