Getting your screen replaced by Apple is easy and surprisingly cheap -- possibly even free, depending on the type of damage and whether you're still under warranty.
Apple charges $109 (plus tax) to fix a broken iPhone 6 screen and $129 (plus tax) to fix a broken iPhone 6+ screen. If you can't get to an Apple store, you can mail your device in to Apple for a shipping fee of $6.95. If you purchased AppleCare+ ($99 for two years) for your device, you'll only have to pay $79 for a screen repair -- though AppleCare+ only covers two incidents of accidental damage.
Finally, if your phone is still under warranty and the crack in your screen is just a hairline (single line) crack, Apple may replace your screen for free, because in certain cases it considers hairline cracks (without any obvious sign of drop impact) to be indicative of defective glass. You can check your warranty status on Apple's website.