Being TEFL certified will certainly make you more marketable and it will also prepare you for the job.
CELTA is undoubtedly a popular qualification especially in Europe but it is also a fairly intensive certificate program in terms of finance, workload and commitment.
Training online is a viable option, many good schools around the world happily accept online TESL/TEFL Certificates issued by reputable organizations. So if you are looking for a way to get some basic training and a foot in the door, online training is an option worth considering.
Just make sure you choose a reliable course provider, and this applies to any course whether it is conducted online or onsite.TEFL Course Review
is a site where TESL/TEFL courses and course providers are independently reviewed and rated by those who have actually taken the course. It might help you separate the wheat from the chaff.