I just bought a new phone yesterday and its screen is scratched

Start here.

iPhone Service & Repair - Apple Support

Did it have the scratch out of the box or are you saying you’re surprised a scratch could happen that fast? You can request a repair but be prepared to possibly be told it’s a cosmetic issue and won’t be “fixed” since it doesn’t affect the device working. There are about to be a ton of scratched screen complaint threads starting soon after Friday most likely as it happens after ever release….the usual suggestions provided (screen protector, case), the usual debate on what can and can’t cause a scratch….etc. It’s a frustrating thing for those that experience it for sure. Hope you can get some assistance with a repair but it’s usually an almost impossible thing to get them to cover.