If you have reached puberty, you can get pregnant, regardless of your age. Yes they can if they have started ovulating. Even if a girl hasn't got her period yet she could still be ovulating. That is what you do before the bleeding starts. There are kids that has become moms at 12. However, it is illegal to have sex under age 16 in most countries.
Ovulation is when a female sheds the lining of her uterus allowing pregnancy. You can tell a female is ovulating by asking or looking for signs. Some of them include: abdominal pain, bloating, irritability, fatigue, even cravins; these and many other rings are possible ways to know. If you are in a relationship with a girl ask her.
Women/girls ovulate about day 14 of their cycle. This is the release of the ovum ( egg) so she can become pregnant. If sex takes place between days 12 and 18 of the cycle there is a high chance that sperm will fertilize the ovum. So, when she is ovulating she can become pregnant. If she does not, then, a period will take place about 2 weeks later.