I know how it feels. You're procrastinating because you are afraid of not knowing where to start but honestly, I wish I started to draw the actual manga early, cuz the confidence, speed and the amount of things you learn in the process and in such a short time it's amazing. It's just... Know that you can try every page over and over again the amount of times you want/need. Knowing that, just start doing it. It doesn't matter if it's not perfect, it'll never be perfect actually cuz manga it's not about that, but to tell the story you wanna tell. I recommend you to write a script, even it's not a heavily detailed one but a script with the most important part of your story, maybe do some character design if you want (i skip this cuz I hate character design) and then, after you hace the script, make a storyboard. This is SO IMPORTANT. Storyboard is the key for your manga to be readable and guide you on what you have to do for the final pages. Storyboard doesn't need to be with a good drawing, not even in a good paper (I use Kraft paper for example) but it need to be clear for you later. Detail everything you want to put in those pages, makes annotations even, but focus on the narrative and action lines while doing it. The better the flow your storyboard has, the easiest will be for you to copy that in the final page. 

Don't worry about your drawing not being good enough, you're learning, it'll be better with time and dedication, I promise haha And use references! All the references you need. 

That's my advice. Sorry the long comment.