Sounds like common sense but let me make it clear.
KEEP A JOURNAL!
A real life, physical book with blank lined pages for you to hand write sentences in. And use it for everything.
Write down that weird dialogue that just popped into your head even though you don’t have a story for it to go into. Write a list of things you want to write stories about. Write poetry. Write prose. Write microfiction. Doodle.
Just do it!
If you fill up a notebook a month you will have 12 BOOKS OF IDEAS by the end of the year. And they’ll all be yours.
You might have access to all those wonderful writing apps I’ve been talking about but try to refrain from dong your primary composition on your phone, tablet, or laptop. Many studies show that handwriting information helps you to remember it, this will help keep you from losing your place in your work.