Hi Freddie, welcome to the Nextion world ...
The Nextion is accepts much of its commands via text over serial
These commands can be found in the Nextion Instruction Set located here
As well as the list of Nextion commands, the Nextion Instruction Set also has
A list of the valid Nextion Return Codes and the format of the expected data being returned.
Other samples and Tutorials can be located in the Itead Blog located here
Further Information can be found in the posts in the Announcement section of the Forum
Nextion Users manual
Most simple start is, create simple HMI's, without any Arduino connect, and play with the debugger. All you send there to the simulated Display, can also be send over serial via Arduino ...
There you can see and observe, how all works together ... and when finally all works inside the debugger, try to port your commands to your Arduino ...