Every good build starts as an irritation, not an ambition. The first step is noticing it out loud.
You do not need a business plan. You need one sentence describing something in your life that is currently held together by memory, sticky notes or a group chat.
Write it the way you would say it to a friend. 'We keep forgetting birthdays.' 'Nobody can find the recipes.' 'I re-type the same enquiry reply four times a week.'
That sentence is the seed of the whole project. Everything later is just detail added to it.
Practice
Write three sentences. Keep the one that annoys you most.
Try this
Say the idea out loud in under fifteen seconds. If you can't, it isn't one idea yet.