The first sentence is always a lie. Not a dishonest one — a necessary one. You write it to convince yourself that you know where you are going, even though you don't. The first sentence is the writer's performance of confidence for the writer's own benefit. The actual first sentence of the piece, the one that will one day open the published work, is almost never the sentence you wrote first.
This is something beginning writers find troubling and experienced writers have made their peace with. You do not begin a piece of writing. You begin a process that will eventually produce a piece of writing. The early drafts are not failed versions of the final draft — they are the necessary excavation work. You are not writing badly. You are finding out what you are writing about.
"You do not begin a piece of writing. You begin a process that will eventually produce a piece of writing."
The advice we give every writer who tells us they don't know how to start: start wrongly. Start in the middle. Start at the end. Start with the sentence you are most afraid of. Start with the image you keep returning to but haven't yet been able to explain. The beginning you need is somewhere in that wrong beginning, waiting.