If you're looking to bone up on your knowledge about teaching English before you head abroad, check out my top 5 books for the ESL teacher just getting started. Of course, it's better to take a course like the Celta, but reading a few books is obviously better than nothing.
How To Teach Englishby Jeremy Harmer
How to Teach English by Jeremy Harmer is a classic and should be the first book you read on the topic. It's extremely well-organized into short sections, assumes that you know nothing and also gives some very practical advice and some activities to help get up and running for your first few weeks in the classroom.
Approaches and Methods in Language Teaching by Jack C. Richards
While more theoretical than the Harmer book, this one can provide you with a solid foundation of the major approaches for language teaching. It's perfect for the ESL teacher who is already confident in the classroom, but needs some knowledge about the specifics of teaching English as a Second Language
Speaking Activities That Don't Suckby English Teacher X
Speaking Activities that Don't Suck by English Teacher X is an excellent speaking games and activities resources for the beginner teacher before they walk into the classroom with nothing. When I was just starting out, I always tried to have a few speaking activities in my folder or in my head that I could pull out if required. This book will help you with that in an interesting, engaging kind of way.
If you're clueless about English grammar like most native English speakers, this book will be your new best friend. Before you walk into a classroom, it's wise to have a basic idea of what you're doing and this book will teach you, but in the least boring way I've ever seen.
How To Teach Speakingby Scott Thornbury
Most newbie teachers will primarily end up teaching speaking and conversation so "How to Teach Speaking" by Scott Thornbury will give you a leg up on the competition. It does a great job of covering the theory, but also gives a wealth of practical advice and information that you can use in the classroom.