According to accepted theories about the origin of the universe within the first few seconds after the big bang most of the physical rules that allow the universe to function as we see it had come into being. After that your question is pretty simple. Suns, that is stars come into being once huge clouds of extremely small particles finally weigh so much that they collapse into themselves and begin the process of fusion. Planets are formed the same way.
Where did the particles come from? Well atomic particles came into being with the big bang, that is protons, neutrons, and electons. These particles can combine to form atoms. Thought atoms larger than Hydrogen require a sun in order to form cause they are created through the process of fusion. These particles of course come together by the rules of gravity around suns to create planets.
The processes of planets are dependant on a number of factors including their distance from the sun, their density and size (which will alter the interior processes), what they are made out of, as well as the composition the atoms within the planet have taken. For instance, Mars is techtonically inactive, but it may not have always been the case. Before the entire surface was covered in continetal crust (rocks like granite, rhyolite, andesite, and diorite) and still had oceanic crust (rocks like basalt, obsidian, and Gabro) subduction could occur, whatever plates and continental materials that were on the surface could move around (like our own plates) and planetary processes such as earthquakes and volcanoes could occur creating things like water and oxygen (specifically O2 what we breath).
The point is that there are explainations to your question. I think a better question for atheists would be "How is it that all these things throughout the universe that work so well together do so by random accident."
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