It doesn't seem that he began his life in mathematics. Does anyone know how it came to be ? What did he read ?
Mauro Allegranza's comment pretty much says it all but to elaborate a bit one could mention that Leibniz came to mathematics rather late in his intellectual career and was essentially a self-educated scholar. His older colleague Huygens encouraged him to pursue mathematics, and his encouragement (on many occasions) was instrumental in Leibniz's development. In a way being self-educated bestowed a certain advantage on Leibniz because it made it easier for him to question some of the reigning dogmas at the time. Specifically this applies to a traditional hostility to indivisibles and infinitesimals that had deep roots going back to the Council of Trent and connected to the dogma of the eucharist. Leibniz was able to shed these intellectual shackles in his work, giving us a mathematics that was original in both senses of the term.