I am currently serving in as an Assistant Plans and Training Officer in HQ, NTC. How can I best prepare for this assignment?

Learning how to index so you can find the commander what he asks for when he wants it. Train a couple of assistants unless you're into sadomasochism after duty hours.

CPT (Join to see) Get an office call with the rater and senior rater and find out what they need done. This sounds like a non doctrinal assignment. Get some initial guidance then research. My gut tells me you'll be somewhere near the Chief of Staff.

Learn NTC terrain, capabilities, and missions of tenant organizations. Something tells me you'll end up involved in major DV visits.

I'm not really sure what your job entails, so my examples may be off, but the principle I suggest is the same. Don't just read, but study the various NTC, FORSCOM and Army regulations that pertain to each aspect of RC participation at NTC that you are responsible for. So if you do something related to the various milestones for RC units attending NTC, then know those milestones up and down. Throughout my career I found that digging into regs was invaluable. First off, many people will say "the reg says this..." or "you can't do that because of the regs..." but often times those people haven't actually read the regs, but they are just passing along the stuff they have heard of how things go. I refer to that as the "myths and legends". They usually derive from a decision someone made long ago that may have made sense then, but doesn't now. It may not even be dictated by the reg at all, as regs change but many people don't keep up with changes. But it is "the way it is done" and no one questions it or looks for a better way.

Also, by knowing the regs, you learn the exceptions that are possible. For example, usually any reg has some sentence like "this can be waived by the first COL level commander (or general officer, or whatever) in the chain of command". So when your boss or the units you are helping want to do something, and others say "the reg says you can't do it" you can inform the higher ups of a way to get it done. Being a good staff officer is about helping the boss get to yes, within the regs. So know them up and down.

Yes, there will be NCOs and civilians who have more experience than you who can steer you in the right direction. But don't just rely on them. Make yourself an expert in the areas you are responsible for. It takes time and effort outside your day to day activities. You won't have time to just sit around all day reading regs. But make the time. You want to be a confident staff officer about who others say "that guy knows his stuff". And confidence comes from competence, and competence comes from knowledge. So take the time to acquire the knowledge by going to the source regs, not just listening to the myths and legends.