The most common way to die from taking a selfie is falling from a heights, followed by drowning.
From Priceonomics:
One-third of all people who met their demise in the midst of a selfie fell from heights — most commonly, a cliff or a building.
In late August of 2015, for example, a 25 year-old man Chinese man ventured off-trail to snap a selfie at the top of Long Men waterfall in eastern China. Distracted by the camera, he took a misstep and plunged 100 feet down into a ravine, where he died instantly. When authorities recovered his body several days later, his selfie stick-mounted phone — still intact — contained a picture of him in the process of falling.
The following month, a 17 year-old Russian student climbed a nine-story building near Moscow, and hung off the ledge to make it appear as if he were falling. He intended to capture the "ultimate" selfie for his Instagram page; instead, his hand slipped, and he fell to his death. Just weeks earlier, he'd posted a similar image of himself in a precarious position: