When starting out writing [and even when established] it is good practice to have a structure in what you are doing when you are writing.
I strongly advise that you write everything down.
If you have an idea or even a part of an idea, it is surprising how easy that idea will disappear as your mind fills with other ideas and fragments.
Carry a notebook with you and write things down even if you are not ever going to use them again.
You can refer back to things and come up with ideas that you had many weeks or months ago which may not have been useful at that time but which may have some use at a future time.
Indeed you can even use previous ideas in order to conjure up new ideas related to that original framework.
You can develop variations on the themes of ideas which you had thought of before and of course you can use those very ideas by themselves.
Getting into a routine of writing down your ideas and thought processes will make you more disciplined in your writing.
It will help you with writer's block, and it will help you to develop subjects and materials for future works.
Once you start writing ideas down, you will soon discover ideas lurking about in the back of your brain that you had never even been consciously aware of.
The ebb and flow of these ideas and the structure by which you pull these ideas out of your thoughts will enable you to conjure more and more ideas of an interesting and productive nature that you can use for your writing exploits.