"She is a girl and I am a boy."
Translation:Ella es una niña y yo soy un niño.
If you follow the lessons in the right order, duolingo has not presented estar to you yet, so you can be pretty sure you always have to use ser at this point :-)
Ser and estar both mean "to be" but the way you use them is different. I think the gist of it is that you use estar if you want to describe a state/condition or a location someone or something is in and ser in other cases. But don't take my word for it. It takes some practice to get it right.
You may find this helpful: http://www.studyspanish.com/lessons/serest4.htm
I have a question, I am Italian but there isn't spanish/italy so I study in english. I try from italian to spanish and niña is child (bambina) in italian and girl (ragazza) is chica! In english "girl" you use it also for small girl? Becouse a spanish told me niña is for max 14th age.
In English, "girl" is generally used for any female who is not an adult (18 years old). Woman is usually used after that age. Although Americans are beginning more and more to treat ppl in their 20s like they are children. So, 20+ year old women may be referred to as "She is just a girl." Sometimes for girls maybe 5 and younger or so we may use the term "little girl" to describe them. I hope that helps.