just make a decision to get the direction set
then you can add more details next
and any time you can revise the whole plot if you dont like what is happening
nothing is ever set in concrete until the publisher prints the thing
but it is harder to make changes the farther along you are
so do get the basics locked down well to avoid needless rework
have you considered a template or other tool to use as a guide?
they are very helpful for new writers.
many free ones on the internet. pick one that seems useful and adapt it.
and tools like dramatica pro are good for experienced writers.