iPhone X - Newly registered Apple ID
First of all, when you create an iCloud account it is also automatically an Apple ID as well. So, based on your question, you now actually have two Apple IDs at your disposal. However, this doesn't mean you need to use both, or that you only have to use one. Let me explain...

An Apple ID can be used to access Apple services such as the iOS App Store, macOS App Store (if you have a Mac), Apple Music, iCloud, iMessage, FaceTime, and so on. So, if you wanted, you could use your Apple ID to sign in to all of Apple's services (i.e. using a single Apple ID and password).

However, you don't have to do it that way. For example, you could use your iCloud account to sign into iCloud on your iPhone X and still use the first Apple ID you created to sign into the App Store on your iPhone X. And you could manage these happily (and simultaneously) on your device. Some people even use three IDs (e.g. a different one for Game Center).

So, based on what you're wanting to do, you can choose to sign out of anywhere you've already used the first Apple ID and then sign back in with the iCloud account you created.

However, there are a couple of scenarios you should be aware of in case they apply to your circumstances:


If you've purchased any apps (free or paid) from the App Store using a particular Apple ID, then you're going to need to enter that Apple ID's password when updating those particular apps in future. One way to get around this would be to delete those apps and then sign out of the App Store. Then, sign back in using the other ID and download those apps again. One negative of this approach is that you'd have to pay for it again if they weren't free.
In the unlikely event that your first Apple ID is attached to one particular country's services (e.g. the US iTunes Store), and the second ID is attached to to another one (e.g. the UK iTunes Store), then you may lose access to any purchases you made using the first ID (depending on the content, as this doesn't apply in all cases). Regardless, this is not something you need to worry about, as this would only be the situation if you deliberately created accounts in multiple jurisdictions for a particular purpose - it's not something that would have happened by accident (unless you purchased the iPhone X in one country and created an Apple ID, and then visited/moved to another country and created your iCloud account there). However, if you were in this situation, then you can still choose to switch between IDs if/when you needed to access/view the other content.


Finally, if you've only just purchased the iPhone X and don't have a lot of data already on it, you could choose to set it up from scratch again and just use the ID you want. If this is the case, post a comment below and I'll explain how to go about doing this.