[noun (place)] + に + [noun (person/animal)] + が + いる [noun (person/animal)] + は + [noun (place)] + に + いる [noun (person)] + には + [noun (person/animal)] + が + いる
がいる says that a person or an animal is there. The one that is there takes が, and the place takes に and usually comes first. いる is the plain form and います the polite one.
いる is for people and animals, anything that moves by itself. Things take ある, and so do plants.
When the person or animal is already the topic, it takes は instead of が and comes first. The place follows.
English "have" a sister or a dog is がいる in Japanese. The one who has them takes は or には.
These go beyond saying that someone is there.
いる is a る-verb: いない, いた, いなかった, and いません, いました in the polite form.