A trilogy is an ambitious project for a beginning Indie author, no doubt. But it is doable.
It's just like eating an elephant...one bite at a time.
Three tips that have helped me along the way during this process are as follows:
1) Keeping notes, either jotted down on paper (a dedicated notepad is kind of important) or on a phone app, such as Google Keep
I started writing notes as plot ideas and crucial elements came to me, that way, when it wasn't convenient to write at that moment, I was still able to capture the mood by jotting down a few key phrases. I have a distinctive pad of paper and I told my family, "If you ever see these papers floating around DO NOT THROW THEM OUT". I am happy to report that they listened, and I will find them stashed on the important paper pile if they get away from me.
Google Keep is my favorite note-keeping app. I like the choice of colors, the block layout that presents on my phone, and it has become a habit to open it up and add notes of things I don't want to forget. Super easy to access and read, and it's great to have one place to go to for keeping up with the ideas that sometimes flood my mind.
2) Outline/Plot/Scheme/Happy Endings
So every author is different. Some have a definite outline that they strive to follow as they write their book. I know a guy who used a spreadsheet...complete with graphs...to chart out the story arc of his book. It was awesome! Other people jump in without looking, (like me!), and just follow their noses to the end.
Whatever your writing style, it is uber important to know what 'Z' is. What I mean is that you really gotta have an idea of what your The End page is going to look like. Even if it is only a vague notion, that vapor of an idea is going to be important in giving your piece direction. So you have your point A and your point B...but make sure your general idea of an ending is somewhat figured out. A destination is a good thing to work toward. Head towards your 'Z'.
3) Pay Attention to Your Genre
Let's be real for a minute. Writers are artists, and we like to create, be our own voice, march to the beat of our own drum etc. But predictability sells. Why did vampires get so big so fast? There was a formula that the authors followed, and the readers ate it up.
So if you're writing dystopian urban fantasy, you might want to pay attention to the best sellers in the genre. What is their story arc like? Do the main characters get separated in Book 2? Does the protagonist have a faith crisis in Book 3? We all want to be different, but there are a few things that readers come to expect in their genre of choice. You might want to give them what they're seeking, if you want to sell a lot of books!
Hopefully these ideas will help you along as you work on your Trilogy. I'm going to get back to '98'! I'm almost halfway and loving it!