Hi Sam!

Definitely better to grab yourself a certificate, especially if you plan on moving around. Some countries or employers don't require it but most do.

Where is up to you, really. If you don't enjoy where you are then you're just gonna feel miserable. Ideally, you would want to take your certificate course in the country you want to teach at.
1. you have the chance to take a look at the school you would want to teach at: amenities, class size, etc.
2. though most do hire via the web, it's generally better if you showed up at the school with your resume and ready for an interview.

Asia is full of job openings year round, but of course it helps if you've got some interviews lined up when you're already there.
You said you have a friend who's taught in Taiwan, perhaps he knows some good schools.
Arriving in between school semesters would be best. Wikipedia is your friend. For Taiwan, read up: http://en.wikipedia.org/wiki/Education_in_Taiwan#Public_education

you can ask around forums if people have had bad experiences with a certain school, although it gives you an idea of who's a great employer, the web is just full of bitter people lashing out at former employees :)

what else can you do to prepare for teaching ESL?
Just... expect a heavy workload in some schools :)