Odds of Getting In
This is determined almost entirely by the district to which you're applying. You don't have to be a rocket scientist to sub.
But just because you put your name on a list doesn't mean you'll get called in. If a district has a shortage of subs, you might be placed in a classroom the day after you apply. After all, it's not legal to have a classroom full of students with no teacher.
However, if a district is well-stocked in subs, the odds of getting called are lower. If you're just starting out, you're going to be working on a trial basis. A district will let you sub for a day, and then an administrator will seriously consider if they want to bring you back.
It may take a while to even get called at first, because districts will almost always give hiring priority to senior subs. They want to entrust their classrooms to people they know, before risking turning them over to someone new.
That's why it's important to make connections with administrators and teachers at the schools—anything to make your name stand out on the pile. Some subs who are starting out will work at the school they attended as students.
Unless they remember how terrible a student you were. Then it might backfire.
It's always advisable to apply to multiple districts, but you can't just apply to every school under the sun—typically, there will be state regulations limiting the number of districts for which you can work. Do your research on this one, because the more districts that have you on the list, the better your odds of getting called.