Let me first say that there are a LOT of task management apps out there. Which one is best depends on your personal taste and how you like to work. Let me also add, that there are equally as many ways by which to organise your tasks. Again, they are all equally valid and you need to find one that suits you and the way you work.
However, the concept of a task management system more advanced than a todo list scribbled on a scraps of paper is new to most writers and I’ve had a few ask if I could show how to replicate what I do.
I use a tool called ToDoist and a method based on something called GTD. It works for me, it might not work for you. BUT, if you’re looking to start, this is pretty simple to set up and run, you don’t need any advanced features so you can do this with the free version of the app, and it should give you some benefit, even if you eventually move to something else. This setup allows you to add habits and track all those writing admin tasks we always forget about (backup manuscript, query agent, organise cover design, write short story before competition deadline, etc.)
This might be the only video you ever need on task management.