First you will need Snapdragon 625 or above processor in your smartphone

Now, click on link which I have added above.

Now, click on Pre-register and that's it. You, registered the Apex Legends Mobile  

Apex Legends Mobile will be available from May 2021 to few users and at the end of 2021, it will be available to for everyone.

Now, if you have iPhone then you have to wait for some time. For now, it's only available for Android Users.

EA has posted some info about Apex Legends Mobile, if you want you can click on above link. That's it.