The filmmaking process is a rather large octopus to wrangle.
First - just go ahead and start whatever filing system you find most efficient & practical, paper or online, and start writing out all the half thought out ideas for different films you have.
DO NOT get married to a single piece and insist that nothing else gets done before completing it.
Some projects are best shelved until a later date and time when you have access to the proper budget/equipment/talent/location resources to do it justice.
Nolan sat on INCEPTION for a decade.
Cameron sat on AVATAR for even longer.
If they can do it then...
Write/direct/produce the ones you can do well, not the one you WANT to do the most.
Second - budget for equipment, costumes, props, locations, cast & crew is the Achilles heel of all creative filmmaker types.
For some irrational reason too many nube filmmakers believe money (representing the labor of others) just pours out of the clear blue sky to fund their art interests/hobby.
Um... no.
Is anybody going to pay for you to pack your bags, fly to the Yucatan, party for a week, and fly home? Nope. Pretty much the same thing.
Therefore, think about the budget/equipment/talent/location resources you actually do have and those you might be able to get and those you would pay for - with the understanding you'll likely be throwing money down a hole just as if you took a vacation - and write accordingly.
Strongly consider both
exactly who the audience is for the project/product - and - where & how your film project/product will be distributed to them to enjoy or be entertained by.
Know your film's genre(s). Know how those watchers will be watching it.
The answers to these two questions will dictate all sorts of considerations and criteria to satisfy as writing and pre-production begin.
Also, justinisfilming is right on: make some practice shorts.
Each one will have different challenges teaching you different things.
You will be better off making five three minute shorts than three five minute shorts, and a lot better off than making a single fifteen minute short.
Good luck.
Have fun.
Everyone's having a blast until someone loses an eye.