While not required across the nation, some states require a bachelor's degree or the successful completion of a competency test in order to apply to become a substitute teacher. Your state's department of education website, or the school district you're applying to teach in, can tell you the specific requirements.
One of the biggest struggles you should prepare for is the inconsistent scheduling that is part of the job. Very rarely will a long-term substitute teaching position open up, which means that you'll often be in one classroom one day and another classroom (or even school) the next. The best way to look at these kind of situations is to just understand that you're getting more practice with a diverse group of students and faculty members that will help your future job prospects.
Organization is another core value that substitute teachers need to work on if they're going to be successful. Being a substitute teacher means following a lesson plan that was laid out for you by the teacher so that he/she can resume their own teaching duties when they return to the classroom. Being organized with your paperwork & with your students is very important.
Substitute teachers need to have patience with themselves and their students. Patience is important because many children don't have the respect for a substitute teacher like they have for their own teacher. They may view having a substitute teacher as having an "off" day and finding ways to motivate children while also being patient for them can be a hard balance to get used to.