I think most people know how to use Microsoft Word for their WIPs (Works In Progress). It's a reliable program available on most, if not all, computers. Most editors appreciate Word Docs because they can use Track Changes to share edits. I still use Word myself to send the final draft of my WIP to my editor.
There are other programs available for use, though, that might make it a little easier to organize your thoughts and/or motivate you. And there are some programs that will literally help you plot out your book and organize your series universe.
Below is a list of creative writing programs, some of which I've used. All are paid programs, ranging from CURRENTLY FREE up to $20+ per month.
As with anything on this site, try them for yourself before you commit to anything.
Scrivener- This is a popular writing program with writers. There is a steep learning curve to use the program, but once you get the basics figured out it can be an invaluable tool. There are many, many tutorial classes and videos on how to use it. It's been my go-to for several years.
Ulysses- Not a bad little app you can use across devices, but definitely does not have all the bells and whistles I'm used to in Scrivener. May be a good thing for certain writers that don't want all the clutter.
Novel Factory- Has a bunch more bells and whistles. I recently downloaded the program to try and it has great graphics and is easy to understand.
Plot Factory- This is a program I'm using myself right now, and I'm really enjoying it. It has all of the bells and whistles and organizational elements you could wish for. As a long-time Scrivener user, I appreciate how easy the program is to manipulate. It also has a very cool feature of being able to narrate your words in male or female voices.
The Quill- Right now the program is free while it's in the beta-phase, but the website says it will eventually charge. I set up a free account and it didn't seem to be as intuitive as other programs.
Wavemaker- again, a free app you can use across devices. It's not as intuitive to use but it does have a great notecard planning tool. I would think about using it just for this, although you can do about the same thing on Scrivener.
4TheWords- I will say this one is very unique. It's more like a game. You have to 'fight' a foe by gaining your wordcount, then you move on. It's vibrantly colored and a bit juvenile, but could definitely be fun.
LivingWriter- This one is interesting. At first glance it looks a bit like a streamlined Scrivener, but it has a lot more functionality. It has templates for several different story arcs. It looks like at some point I myself had a subscription to the service, but cancelled it for some reason. I re-upped it to check it out.
Bibisco- This is a new one to me but it looks to be a strong word processor, with guidance on how to deep dive into your characters and situations.
SmartEdit- Looks pretty basic compared to some of the others I looked at, but definitely usable. It has a free trial.