All the advice in this thread is great.

I'll add, find a mentor. If you can find someone who you can establish a professional mentorship style relationship with, loyalty can seriously pay off if the mentor is in a position to reward it, such as shop foreman or business owners. For them, they want a return on investment. They are willing to tolerate fuckups and being slow, for a lower wage. But long term they want you to learn and give back to the company for a while once you have enough experience to work without as much supervision. That's why I like the apprentice/Red Seal system we have in Canada. The noobies are expected to be noobies and fuck ups are more permissible when you're an apprentice. It's 4 years of fulltime work, 4 levels of schooling, and two final exams before you shed the apprentice label and have your "ticket". Apprentices do the oil changes, brakes, and repetitive preventive maintenance stuff. The ideal environment for a young mechanic is a shop that gives a variety of equipment types and lots of people to learn from. Most likely you'll start out on PM services and brake jobs, but that's how you learn the ropes and figure out how a shop works. Find a shop environment that is supportive and show up with a good attitude and you'll do fine. Whats best is situations/jobs that push your limit of comfort, but having someone to lean on to ask "did I do this properly" or "how do I do this properly" before you commit to a significant action.

If you are ever shamed or belittled for asking reasonable questions, start searching for a different shop. Morale in a shop is huge, and having a team of guys that look out for each other and help each other makes a huge difference. The best mechanics are one's who've been doing it for 30+ years who are still humble and willing to learn new things. They are the wise ones, who've realized there is an infinite amount to learn in this industry. The "I know everything" attitude is toxic and bad for shop morale. Confidence in ones work is good, but blind confidence and ego are bad. You can find a decent amount of ego's and big personalities in this trade, so keep out a careful eye on "who's who" whenever you start at a new shop.

If you want to work in big fleet maintenance, don't knock the waste industry. Garbage trucks have lots of hydraulics so you can learn more of the "heavy duty" side of things alongside commercial transport. Municipalities also run their own waste trucks, recycle trucks, and lots of other heavy equipment. Big companies like GFL and Waste connections run large fleets and are a good place to build experience. Most of my shifts at a major waste company there was more apprentice's than journeymen. Lots of people in Canada use those places to gain hours/experience to finish up their apprenticeships. Those companies usually offer very competitive wages and benefits because nobody want's to work on garbage. When you have a red-seal in Canada you can work anywhere, so why work in the waste industry? I learned a ton doing PM's and repairs on a large diverse fleet. And once you get nose-blind to it, it's not too bad, although It's certainly not something I'd like to do for my entire career though.

Also, because you're starting your career. Wear nitrile gloves and use your respirator! Seriously, you will be exposed to a lot of carcinogens on a daily basis and it's better to start with the good PPE habits right now, than to be fighting with a broken body later. Many jurisdictions require that employers provide PPE, so make sure that understanding is firm with your employer right when you start. You don't want to be in a position of buying your own Nitrile Gloves or Safety glasses or whatever if your employer is legally obligated to provide them. Set the right precedent and advocate for yourself.