There's a couple of different ways that can play out depending on how you present yourself. Most proper tradesmen have a very dim view of trade school, I used to always complain that after you sent an apprentice for another block of trade school you'd have to spend the next two weeks training bad habits and dangerous practices back out of them.
Those that can do, do do. Those that can't teach at TAFE.
So completing a trade school certificate and trying to use that as a qualification will discourage people from taking you on. However, if you instead present yourself as someone who is so determined to get in to a trade that you started putting yourself through trade school shows initiative. Most tradespeople won't take on an apprentice any older than 17 because after that age people's mindsets change and it's much more difficult to teach them, but if you show a determination to learn this will impress them and encourage them to give you a go.
With physical trades an apprenticeship takes 4 years, and for another 4 years after that you would be considered to be of Journeyman status and not taken seriously as a tradesman, after that you can start earning real money.
Since everyone is from all over, I'm just going to note for those unaware that Outdated is from Australia.
If I may present a Canadian perspective, here, many employers only want to hire journeymen and refuse to take on apprentices, due to a variety of reasons, one major reason being the cost/investment in training for someone who might end up being poached by another employer or otherwise leave.
As such, for those who do take on apprentices, in certain fields, employers will quite often hire someone who is already in the apprenticeship system (e.g. minimum of "level 1" completed).
What that means is that someone wanting to enter the field would need to take a "pre-employment" program for several months which is intended to simulate the basics of working in that field, and then complete the level 1 training before finding an employer to take them on as they continue on the apprenticeship journey.
Something to consider is whether a trade has a union hiring hall. Where there's a union hiring hall, it usually makes employment much easier, since you just sign up, and are offered employment opportunities based on seniority, and there's no interviews or risk of an employer rejecting you for arbitrary reasons, though of course usual considerations (such as passing drug exams, where applicable) would still apply.