CELTA is not quite as intense as many people like to say- but it is a 9-5 commitment for a month with approx 1-2 hours of additional work each evening. You don't need any experience to do the course.
CELTA is good preparation for teaching- no online course can prepare you as well as practical experience in the classroom (which CELTA provides). It is a minimum standard though and you should (hopefully) expect to improve on the job. You don't need CELTA to get a TEFL job, but it helps. Above all, employers who require CELTA qualified applicants are generally likely to be more reputable than those who will take unqualified teachers- this is better for the students and also for you (reputable employers generally pay in full, on time, and treat their staff better).
However, if you are planning to teach in certain Asian countries (notably Korea), CELTA isn't as useful as in other parts of the world as it is rarely a requirement for positions.