You're at the beginning of life on Earth... you are a bubble of fats, held in place by proteins, containing some salty water and the amazing molecule, DNA.
You have genes within your tiny, circular, single chromosome (called a ‘plasmid’) which can process the simple sugars which move easily through your fatty wall and turn the energy into copies of themselves.
At this stage, reproduction is still very much at the gene level. There are so few genes in existence, with such general functions, and such a thin cell membrane, that it doesn’t really make sense to describe you as a reproducing set of genes, an organism.
Sometimes your bubble splits and a selection of those copies gets separated, creating a new ‘you’. Sometimes your bubble bounces into another and you merge. Sometimes the copy process is disturbed, and a new gene is created.
Eventually, one bubble accidentally creates a gene which makes a simple cell wall form, and although it reduces the rate at which sugars can enter, it means that this particular set of genes get isolated and can create copy after copy of themselves without merging in any other set. This is the point at which we can start thinking of a single organism… it is called a prokaryote*.
Slowly sugars enter your cell, and slowly you utilise the energy they contain, make copies of your dna and reproduce. It’s a relatively long-lived form of life for this era. You get to make many copies of yourself before eventually some bad luck takes you out. You still, on occasion, swap genes with others around you, and sometimes there are errors in copying, both of which create new organisms.
In order to create the most copies of yourself, you need to make efficiencies in the way you process the available energy. There are other energy sources which can improve you: the Sun, and other chemicals in the soup. If you use the Sun’s energy, you can create the sugars you need right inside you. Otherwise you can be more of a generalist about which chemicals you can consume. After all those chemical pathways are close to yours, so only a small error might be enough.
* This isn't entirely true. There's another family called Archea which has a different type of cell wall. However, it's only a few years since we realised they aren't bacteria, and so their evolution is incredibly poorly understood.
For a good discussion of the current state of Archea, try Wikipedia. If some research comes up that changes what's been written here, then I'll bring it up to date.
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