Looking to join the vocational training program in Germany

Vocational training (apprenticeship) and work (employment) are two different things in Germany.
The former will (depending on which trade you learn) barely pay you enough to survive, but in the end you'll get a certificate and, if you prove your worth, a job at the teaching company or elsewhere.
However, in both cases you first need a company that accepts you (as apprentice or employee), before you can apply for the appropriate visa. To find a job (unless you have rare niche skills - and selling used cars isn't one of them) and even more so to find an apprenticeship, German language skills are key. Thus you may want to improve to B2 or better C1 level before you apply.
To learn a language, speaking practise is required. Thus instead of online courses, you should attend an in-person class (e.g. at the Jakarta Goethe Institute) or visit Germany to attend one for a few months - as in Indonesian friend of ours did recently (while trying to enter a German university).

Thanks for the advice! I looked it up and there's a Goethe Institut in Surabaya, my city. I'll  pay them a visit and see what classes they offer.