The process where ancient organisms get preserved and become part of the fossil record is known (not surprisingly) as fossilization. You may use this as a key word and do a search on the web sometime.
Basically there are different means of fossilization. I will give you two examples:
1. Imagine a leaf falls off of a tree and lands in a lake...The leaf sinks to the bottom of the lake and may get quickly covered by oxygen poor mud...
The leaf can leave an impression of its shape in the mud and then long after the plant material has 'evaporated' the impression of the leaf in the mud remains.
2. Another example is called "replacement". In this method of fossilization you should think about a bone...say your leg bone! If it gets buried by sediment and then over the years solutions of ground water will flow through the bone. The minerals in the bone (apatite and calcium carbonate) can get replaced by silica.... the replacement takes place atom by atom and although the "bone" eventually is made up of silica, (i.e. all the original bone material has been replaced) the form of the bone remains the same!These are just two ways that fossils can form. I suggest you look up fossilization in a text book on PALEONTOLOGY, the study of ancient life to learn more.

Old bones, wood, and other harder organic matter form into stones when water carrying dissolved minerals seeps into small holes in the bones, wood, etc.Over time, the minerals can crystallize (solidify) and fill in the holes with rock material. At the same time, the old calcium that was in the bone wears away (dissolves). Then, the bones, wood, etc. turn into stones.


A cool place to see this is in Arizona at Petrified Forest National Park where fossilized wood is all over the place. It's really neat!