If we define "Christianity starting" as "the time when the belief in the divinity of Jesus became the foundation of a new religion," then it began very shortly after his historical appearance---the middle of the 1st century AD/CE. If we define it as "the time when the followers of said religion began identifying themselves specifically with the label "Christian", as opposed to other labels like "the brethren" or "the Way" or "The believers"," then it's a little less certain, although the religion is mentioned by name by Pliny, c. 100 AD/CE, and Lucian, c. mid-2nd century AD/CE.