There is a LOOOONG process for this so here it is;
First things first.The animal dies. Soft parts of the animal's body, including skin and muscles, start to rot away. Scavengers may come and eat some of the remains. Before the body disappears completely, it is buried by sediment - usually mud, sand or silt. Often at this point only the bones and teeth remain. Many more layers of sediment build up on top. This puts a lot of weight and pressure onto the layers below, squashing them. Eventually, they turn into sedimentary rock. While this is happening, water seeps into the bones and teeth, turning them to stone as it leaves behind minerals. This process can take thousands or even millions of years.
