Screen Time lets you monitor and control your and your family’s device usage, and is controlled from Settings > Screen Time.
There are two things you should decide right away:
Do you want to set a Screen Time password, which must be entered to change Screen Time settings or override limits? If you’re the only person to use that device, it’s probably worth bothering with only if you need additional friction to enforce self-discipline, but if you have kids that use that device it might be a good idea. You can set that with Settings > Screen Time > Use Screen Time Passcode.
Do you want to share Screen Time settings across devices? That’s enabled by default in Settings > Screen Time, and probably makes the most sense, ...