The short answer: Money and time.
Everything else seems to be up to you: if you’re getting certified online, you’ll need internet access. If your taking an on-site course, you’ll need to proper stamps and tickets to get to the course location, and once there you’ll need to find food and shelter.
Courses cost anywhere from $900-$2500USD, depending on exact certification you’re getting and if your taking it online or on-site. The on-site courses I’ve been considering in Prague all seem to be around €1300 ($1500USD).
Course length can vary widely as well – from 20 hours to 250 hours, or so I’ve read. However, the general standard recognized by most schools is the 120-hour certificate, with at least 6 hours of observed teaching practice. At an on-site course, the 120-hour certificate is usually a 4 week program.
I found at least one source that said most courses require a degree, but many of the courses I looked at mentioned no such requirement – make of that what you will.
Most courses require being at least 18 years of age, and some require 20 or 21.
Most courses require either being a native speaker of English, or passing a proficiency exam if not.