Hi,
Having been an EFL teacher and training I would strongly advise against doing an online TEFL course purely for the reason that you are showing some concern - you will benefit immensely from hands-on practical experience in a classroom with real students.
There are a wealth of courses available and as I am sure you must have realised by now, CELTA and Trinity are the 2 industry-wide recognised certifications. That does not mean that there are no other genuinely valuable courses on the market, however you will be guaranteed a level of quality through a centre offering this training. Alternatively contact a centre that you are interested in and ask to be put in contact with past trainees who will be able to give you firsthand knowledge of their experiences. If the company does not provide this then the likelihood is that their product is not what you are looking for.
Good luck to you and I hope you enjoy many fruitful, happy years interacting with a multitude of students who will enrich your life in so many ways.
Tuppence
ps Just in case, let me warn you that the initial training is incredibly rigorous but if you are truly interested in teaching then it is most definitely worth the time, money and effort to gain the basics of classroom life.