GoKawiilThere's an art to picking a good alarm sound. It can't be too pleasant that you happily listen to it for a few minutes instead of doing what you're supposed to. Nor can it be too annoying, as that can put you in a bad mood as you scramble out of bed to shut it off (you do keep your iPhone out of reach while you're in bed, right?). Some of the default ringtones and alarm sounds on iOS can veer in the latter direction, unfortunately. Thankfully, it's relatively easy to set up a custom alarm sound on an iPhone.
If you'd like to use a certain song, the easiest way to do that is to be an Apple Music subscriber. The service has more than 100 million songs and you can set one as an alarm sound. First, you'll need to add the song to your library. To do that, open the Apple Music app, search for the song you want to use as an alarm and tap the plus (+) icon next to the relevant result to add it to your library.
Then, all you have to do is open the Clock app on your iPhone and tap the Alarm tab at the bottom of the screen. You can either tap the plus icon to create a new alarm, or select an existing one. From here, select Sound > Pick a song. Search for the song you'd like to use as an alarm, tap the plus button next to it, then tap the back button, and finally Save or the check mark in the top right corner. That's all there is to it.
If you'd prefer to have an entirely custom alarm sound on your iPhone, though, you'll need to take a different approach.