You have been given a range of views in this thread. All the views expressed have valid comments. I did CELTA in the UK with a university Centre for Foreign Languages. It was an excellent course, but yes, it was intensive, even for me with years of experience in teaching and training and a sound academic background. There were students on the course who were teachers and one dropped-out. Yet at least a third of the group had never taught before and all passed, some with 'distinction', so it depends on your motivation. Be prepared to put your 'life-on-hold' for one month. As for getting started, just buy some books and get studying before signing-up. Remember that the "A" in CELTA stands for 'adults' , for so if you want to work with youngsters you might want a more specific TESOL course.

Two books I would recommend:-

SWAN, Micheal and WALTER, Catherine. How English works: a grammar practice book with answers. Oxford University Press. 1997. (reprinted 2008,2009.)

HARMER, Jeremy. The practice of English language teaching. Longman(latest edition, 4th). This one is on the CELTA reading list.

Good luck.