God. Allah. Yahweh. Parvardigar. Elohim. These are all different names for a god believed in by lots of different people and religions. But there are also lots of other gods as well, each with a different name and group of followers around the world.
Not everyone believes in god, but for those who do, they believe that he (and god is invariably described as a “he”) is the one responsible for the creation of the universe. Yes, the universe and everything in it – humans, planets, stars, all of it.
People believe different things about god, though; they believe he said everything in the bible, or the koran, or the book of mormon, or … well, you get the impression. There are a lot of different interpretations of god’s words, so it depends which one you believe; you usually believe in the same god as your parents, although sometimes people change their minds and move from one religion to another.
Some people, though, don’t believe in god at all. I don’t, for example, because I’ve not seen any evidence that he (or she) is real. People who don’t believe in god are called atheists, although we often call ourselves humanists as well.
Some people will try and tell you what to believe; they’ll expect you to think the same thing as them. That’s not fair or right; it should be down to you to choose what you want to think. Ask questions, for sure, and do as much research as you can, but don’t be pressured into believing things just because people want you to; remember that it’s okay to make up your own mind.