Fire is not a form of matter; it is a chemical reaction. Fire is not made of any actual materials, but instead it is the result of chemical reactions from heating certain materials while they are in contact with oxygen.
Fire comes from the reaction between oxygen in the atmosphere and a fuel, such as wood or gasoline. When the fuel comes into contact with oxygen and is heated to its ignition point, fire is made. A typical wood fire is created from heating wood to a high temperature with something like a match, focused light, friction, lightning or material that is already burning. When the wood heats, some of it decomposes, and it is released as volatile gases or smoke. The rest of the material is made up of char, which is carbon and ash, a mix all of the minerals in the wood that cannot be burnt.
When the volatile gases or smoke are hot enough, the molecules break apart and the atoms recombine with the oxygen to form water, carbon dioxide and various other products. This reaction is known as burning. These chemical reactions create a lot of heat that sustains the fire. Fire is not really made up of things but instead is a result of chemicals coming into contact with one another.