Hey,
First of all, breathe. Don't spiral out of this, especially since it's your first time writing.
Then some tips:
-Take a note somewhere if you have a sudden burst of inspiration
-Use something to brainstorm
-Don't force yourself to write when you're blocking on something.
I'll tell you - if you check my profile there are two stories. The Nameless is a side project without constraints that I randomly fill (mostly unpublished drafts), when I want to do something more... free. SMD is a fanfiction under several constraints (no change to canon, my OCs have to layer in the gaps without altering anything, need to keep track of each and it's nonlinear, spread on several timelines), and that's why I have a side project, when it gets too overwhelming (I have started this project with the purpose of doing something different and I know I'll damn myself if I don't end it, but I can't force myself either or that will just not flow well) or when some scenes are too hard to write (either choice of words or mentally draining).
Which leads me to another point:
-Know your limits and expectations
I know that I won't be read much, that many will avoid as soon as they see the synopsis, that my style is dense, layered, contains lots of symbolism and figures of speech and many would find this appalling, but I don't care. Which leads me to the most important point:
-Write. For. Yourself.
Don't write for people, just, do what you want to, write how you feel like. If someone hates it, they know how to skip it. If someone likes it, they'll let you know.
Good luck!