My number one advice is to UNLOCK THE BOOTLOADER if possible. Do this NOW, before you customize anything, because unlocking it will ERASE the phone back to factory state (more or less).

Unlocking the bootloader is usually a requirement before a phone can be "rooted", which is something you WILL want to do eventually, so best to unlock it now. On a Google Nexus or OnePlus phone, this is as simple as typing "fastboot oem unlock" on a connected PC.

The downside of unlocking, is that it makes it easier for a thief (eg. USA border guard) to access the contents of the phone. You decide.

"rooting" an Android phone sounds scary, but all it really means is "installing a sudo command (app)". This is just like on (eg. Unbuntu) Linux PCs, where one is frequently typing "sudo" in front of privileged commands to run them as root. Except the Android equivalent is a lot more convenient.