iPhones (and some Android phones) no longer come with chargers (power adapters). If you have a power adapter (and the cable) from an older phone, you can use that or if you have any USB-C power adapters or devices you can use that with the new cable that came with the new phone. If you need to buy a power adapter, see: Buy 20W USB-C Power Adapter - Apple
Also see the following from https://www.apple.com/shop/buy-iphone/iphone-se
As part of our efforts to reach our environmental goals, iPhone SE does not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.
We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones which are compatible with this iPhone. But if you need any new Apple power adapters or headphones, they are available for purchase.