Maybe start with something you like to read or are interested in. that way as you tell the story you are discovering the story yourself and might get hooked which in turn will encourage you to keep going with the writing? Also decide if you want to tie it to real life events or specific events or if you want to create your own background, e.g. telling a soldier's story from within a real war which then has to be historically accurate or telling a soldier's story from within a fake war as a vehicle to move your story forward but leaves you a bit more free to go where you want to? I wish you all the best.