Water, like all matter, is made of molecules, which are in turn made up of even smaller parts called atoms. A molecule of water is made up of two hydrogen atoms and one oxygen atom. The scientific formula for water is H20.
Here is
a diagram of a water molecules. Each large red ball is an oxygen atom and each small white ball is a hydrogen atom. In this diagram, there are 5 water molecules shown.
This infomation is from Encyclopedia Britannica at http://school.eb.com/levels/elementary/article/390625 , which is password protected, and the diagram is from Wikkipedia at https://en.wikipedia.org/wiki/Water.
How - and when - the oxygen and hydrogen atoms combined to form water molecules is a question that scientists are still debating. For all the details, read this Smithsonian article at http://www.smithsonianmag.com/science-nature/how-did-water-come-to-earth-72037248/?no-ist or borrow a book about water (look for the call number 551) or about the Big Bang theory (call number 620).
Please ask for help finding a book on this interesting question on your next visit to the LC.