well F1 is a very stand alone paper , so it doesn’t really matter which one to take. I would suggest F2, simply because F3 is a bit harder and F1 is also not so easy. While F2 is the easiest 🙂
And if u mean, what to do : well get your books , revision kit ..download syllabus, see pilot paper , and all other resources that ACCA propose. Decide about your exam date if u sit CBE , make your study plan and prepare 🙂