[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
います says that a person or an animal is somewhere: "there is", "is in". The one who is there takes が, and the place comes first with に. います is the polite form; いる is the form you find in a dictionary.
Anything alive that moves by itself takes います: people, animals, birds, fish, insects. Objects take あります, and so do plants.
When the person is already what you are talking about, they go first with は, and the place comes after.
います changes like any ます verb: いません, いました, いませんでした. "Nobody" is だれも with the negative.
Japanese uses います where English says "have" a brother, a friend, a pet: the person "is there". The one who has them takes は, or には.
These go beyond saying that someone is there.
いる is a regular る-verb.