While we don't really know the answer to that question, it is hypothesized with good supporting evidence that the first organisms on Earth were probably single-celled prokaryotes that were probably genetically similar to the organisms belonging to the domain Archaea.

Well, we don´t really know... but there are some very interesting ideas about what the first living thing on earth might have been. First of all, we need to understand how Earth was able to sustain life in the first place. Earth was able to support life only after the planet had cooled enough for a rocky crust to solidify. Before that, Earth was covered in a layer of molted rock and lava - not a place for a living thing! But once the earth cooled, water vapor from volcanoes condensed in the atmosphere, fell as rain, and collected on the Earth´s surface, eventually forming the ocean. Besides water vapor, volcanoes also produced gases rich in the basic ingredients (the elements, in fact which you will likely learn a lot more about in your chemistry class next year) of life: carbon, hydrogen, oxygen, and nitrogen. Toxic gases such as ammonia and methane were common. At this point, Earth's early atmosphere consisted entirely of these volcanic gases, and there was no free oxygen. In the primordial "soup" of the early seas, organic molecules concentrated, formed more complex molecules, and eventually became simple cells. But when did these simple cells become simple living cells? That is the real question...

The transition from organic molecules to living cells could have occurred in several different environments. Small, warm ponds are one possibility, but recent work has suggested that deep-sea hydrothermal vents, such as those found along mid-ocean spreading centers today, may have been the site at which life on Earth began. Although scientists have not succeeded in creating life from organic molecules in the laboratory, they have reproduced many of the intermediate steps, further supporting this theory.

So what were these first living things? Studies of genetic material indicate that a living group of single-celled organisms called Archaea may share many features with early life on Earth. Many Archaea now live in hot springs, deep-sea vents, saline water, and other harsh environments. If the first organisms resembled modern Archaea, they also may have lived in such places, but direct evidence for early life is controversial because it is difficult to distinguish between complex inorganic structures and simple biological ones in the geologic record. The oldest evidence for life may be 3.5- billion-year-old sedimentary structures from Australia that resemble stromatolites. Stromatolites are created today by living mats of microorganisms (mostly cyanobacteria, or blue- green algae). These primitive organisms trap thin layers of sediment with their sticky filaments and grow upward to get light for photosynthesis. Modern-day examples of stromatolites can be found in waters off Australia, the Bahamas, and Belize.

Nobody knows. There are lots of theories, but they're all very difficult to test. My personal favorite is organic molecules replicating using grains of clay minerals as a catalyst, which has been observed in an uncontrolled fashion in the laboratory, but only by using molecules already produced through biological means.