0. Please put the topic of your post in the title so that people will understand what they're clicking on!
1. Even if you never have any intentions on releasing the voicebank, I concur with the previous posters that you should start with a simple Japanese CV bank just to understand the basic concepts behind how a voicebank is made. You'll be able to learn:
- How to set up a folder and make sure it's read by UTAU
- How to make character.txt and readme.txt
- Basics of configuring the OTO (editing the parameters, setting an alias)
- How to get sound from your voicebank
- Using your voicebank with pre-existing USTs
A lot of basic essential skills! An English voicebank introduces a lot of other concepts, so handling everything all at once may be difficult, even discouraging. UTAU is a very fun hobby, so I would encourage you to take it step by step in order to understand it more fully.
If you've already got a ton of experience with using existing voicebanks, you're absolutely free to just go ahead and make a complex voicebank. Chances are, at that point, you'll already be familiar with how those voicebanks are set up. Even as a total beginner it's massively helpful to have something finished as reference.