How did life begin on earth according to evolution?
Evolution only describes how life changes, not how life started.
Evolution means change. That is, change of genes/phenotypes over time. It describes how life changes and adapts to its environment by selecting 'better suited' individuals which pass their genes on to their children.
Biological evolution really relies on three axioms: Living things can pass genetic information to their offspring (by sex or asexual reproduction); Genetic change happens (through sex or mutation or both); and less suited indivituals are more likely to die before passing on their genes.
The question 'but how did life start' is not answerable by evolution, that biological theory can only describe how life changes. There are hypotheses (not coherent enough to be full scientific theories though) on how life began. The leading is called the RNA world hypothesis which claims that before DNA, the world was full of replicating RNA mollecules. Since RNA is made of amino-acids and the universe is plentyful in Amino-acids, the idea looks plausible, at least.
Nevertheless there are several significant question in how (and where) life originated which we're still unable to answer. And when we do, it will be the origin of a complete theory of abiogenesis; completely unrelated to the current theory of evolution.