First things first, assess the damage. It might only be a single crack but it could be a full-scale shattering. You need to act fast as cracks tend to worsen over time and any gaps in the screen, no matter how small, expose the inside of the phone to water, debris and toxins.
Check whether your device is still within warranty before you let anyone touch it; there may be a chance you could get a free repair from the manufacturer or through your service provider. If not, you need to think about where to get it repaired, or if it’s even worth getting a repair. It might just be time for a replacement.
You can find more information on fixing specific models here.