Hi Dave,
Welcome to this fascinating hobby (and to this forum, indeed!
).
The first thing to consider is effectively what you want to do with your truck, but normally there's nothing you cant' rebuild if you make mistakes.
When you buy a kit like Tamiya tractor, you get all the mechanicals pieces for a running truck, including a motor. After that you'll also need a battery, an esc to control the motor and a remote control (which is normally include a receiver...). If you don't have any other rc, you must know that you also need a battery charger.
That's it for a basic function model. That kit comes unpainted, so you must consider a paint job on it. If you want to add realism with sound & light, you can add an MFU, or a lights kit (Tamiya sell both of them, specifically designed for their trucks, but it's costly). But if the basic kit can be use with a 2 channels remote, a kit like the mfu needs at least a four channels for controlling all functions.
After that, it really depending what you want to do. If you want a real working truck for example, you must consider swapping brass bushings for ball bearings and change the pinion of the motor.
It's really the basics things to consider at first. You will see on this forum a lot of infos about how to customize your truck for a great looking build. It's easier than it may seems at first.
Hope this help, sorry for my basic english... Feel free to ask if you have other question!
Keep on building!
Gaetan