If there is one thing I think I can advise you on, if there is one piece of advice you take away from this blog and keep stored away in your hearts forever, it is this...
Don't get sidetracked by worldbuilding and plot-creating and character unveiling and forget to actually write your novel.
I have found that as I write, many of the little details that I didn't plan out tend to fall into place. My characters develop themselves, my world builds up around me and my plot (which was just little plot points on a page) turns into a fully functioning novel. That wouldn't happen if I spent all my time planning these things and never moved on to writing them.
That doesn't mean I haven't spent some time planning them. In fact, before writing "The Criminal" and even before writing the first draft of "The Servant" that will never see the sun before that, I spent multiple years doing exactly that: planning. I thought through the characters I wanted in my seven novels. I thought through the major plot for the novels as a whole the plots for each novel. I made maps and family trees. I wrote about my characters' strengths and weaknesses, and fears. I didn't write a word of my actual novels but I wrote plenty of other things.
Most of it got trashed. I remade the maps when I decided to change one of the kingdoms (Snow White's kingdom) to a different kingdom entirely. I remade family trees to include new characters I had added. I zoned in on what I felt was most important to know about my characters and I focused on figuring that out.
All this was happening while I was not rewriting "The Criminal." Because I spent so much time doing this, I left my book in the dust, not working on it at all. However, working on the worldbuilding did get me out of a rut I had been in writing-wise. While I had not been writing at all for a year (aside from the break I took from that not-writing to write my poetry book), working on the worldbuilding helped me to get excited about my book again.
Now, I have an entire worldbuilding binder. There are pieces missing that I hope to someday fill in but I have the essentials completed and I use it consistently to help me as I write and edit. While I stand by the advice I gave in the first paragraph (do not get so sidetracked by planning that you never actually write) I also suggest worldbuilding enough that you know the world your characters live in, can fall in love with it, and use the love to inspire your writing, and can return to your notes on the world when you are unsure while writing.
To kick you off and keep you on target (so you don't go overboard) here are the major things I included in my worldbuilding binder:
economics - My binder is divided into a number of sections for easy access and perusal. I can find what I am looking for within the different sections (a section for each of the four kingdoms, one for the forests and Isle of Meermin together, and one for notes) by looking in the section's table of contents. The pages on economics are located within the "notes" tab because they apply to the kingdoms altogether but for you, that may be different. Perhaps you have 2, 3 or 4 kingdoms each with its own economic system. Perhaps you have 3 kingdoms and two share an economy while the other uses its own. It's totally up to you. Whatever you decide, I do recommend having this page as it is one I most commonly refer back to in my writing and editing. The tells me what markets are like, how selling goods generally goes down, that my money is all coin (and where it is most often kept) and what it looks like. And most importantly: what the amounts are called.
magic system - This is another important one to have and another one I keep in the "notes" section of the binder. For any fantasy book with magic, you should have a good idea of what magic can and can't do and who can and can't use it. For me, these pages (which have not been entirely filled out) tell me the creatures who can and can't use magic, the exceptions, the rules of magic, and the limitations on each separate creature that has the capability to use magic. It is like Sarah J. Mass's "A Court of Thorns and Roses" series. The different high lords have different magic and different capabilities. Mass must keep track of them all and their separate limitations. Do all your magic users have the same magic and same limitations? What are the rules? Is there anyone who can't use magic?
flora and fauna - This section, kept also in the "notes" section of my binder as the flora and fauna ranges from kingdom to kingdom (some occur in all kingdoms some occur in only one) is important to me but may not be important to everyone. I use a number of different magical plants and creatures in my book and it is helpful to me to keep track of them in case I want to go back and use them again or in case they come up some other way. While I keep track of my creatures (werewolves, meermin, witches, etc.) in my characters binder, I keep track of flora here with an inspirational picture or two, the name, and the qualities of the flora. There are some that I wrote in before re-writing the book and there are some that I wrote in as I needed them for a specific scene and as I wrote the book. This section, I feel, is not something that necessarily needs to be done before the book is written but rather is something that should be available as the book is written as a place to keep track of flora/fauna that is added. While you should have some idea of the flora/fauna you want in your world beforehand, adding them in as you write shouldn't be a problem so long as you keep track of them.
government - Ah...finally, one of the things I keep track of within the sections for each of my kingdoms. My four kingdoms each have their own government so it was important that I wrote some notes for each of them on the subject and these are kept within the sections on those. I don't feel that government is as important as some of the things listed above and I do not return to this page in my binder as often as some of the others but more than anything, knowing how the world runs is important. It is background information that may or may not make it into your book but without that knowledge, there might just be a piece of your book missing. Depending on what your book is about, this section can be short and sweet. You may just need to know who is the head of the realm (a king? a queen? a set of people?). But you may need to know more if the government plays a larger role in your book such as if one of your characters is a king, queen or other element of the government.
hierarchy - Hierarchy is incredibly similar to government. Who rules it all? Who has all the control? Who has the money? Who does well in the society? Hierarchy is also something that changes kingdom to kingdom for me depending on what is valued most (merchants? farmers? traders? guards?) so I have pages on this for each of my four kingdoms. I think there are two things that can come from developing your hierarchy: 1. You will figure out where your main character and the other characters in your book fit in society and can develop them around that role and understand how their personality develops because of that role. 2. You can develop the roles that each member of society plays in the kingdom. A hierarchy can just be a list (first there is the king then the queen then the guard then the servant and so on) or it can be more thought out. For example, you can explain how farmers don't make much in certain kingdoms and have to sell their land, becoming tenant farmers. You can explain how bankers or textile-sellers or shoe-makers or whatever it may be are the most well-liked because, in that land, their goods are the most important. I don't often return to the hierarchy page except to remember the hierarchy of the guards but the hierarchies do set up a "feel" for the kingdom which I feel is incredibly helpful.
geography - The final thing that I have included that I feel is important and which I return to for aid on occasion is the geography notes I have included in each section including the section for the forests. These are not comprehensive notes and I don't suggest you lay out every little detail of the geography of your realm. However, I do suggest (particularly if you have more than one kingdom, sector, or whatever you may call it in your realm) writing a few notes about the geography and potentially supplementing them with photos (mine, I take from Pinterest). I usually note things like whether the area is good for farming; mountainous, hilly or flat; covered in rivers and lakes or dry; forested or empty, and more. I also like to make notes about the cities and the major castle (especially since I have books that will have multiple scenes set in the castles). I note things like whether the homes in the cities are good quality or not, whether the people are cramped or more spread out, what the grand entrance of the castle looks like and where it is built and more. It is totally up to you what you want to add here and a picture is worth a thousand words so as much as I find this section helpful, I also find the Pinterest boards I have for each of my kingdoms perhaps even more helpful and would suggest gathering inspiration there as well.