If All Types hires under 21 there is a reason!
Actually, they thrive on ignorance.
Look for some kind of retail thing. Marketing, i.e. putting up displays in stores for companies. I don't know what you would look under, but you would go to different cities and put up new, or revamp old displays.
Ask at Grocery stores about this kind of thing. Maybe the managers can give you ideas.
It isn't the manager or employees that get those things to the store to be stocked. Card companies and florist bring in their own stuff.
So go to all kinds of stores and ask questions, inc truck stops.
There's Auto / Napa, Advance, etc.
Chips, bread, Linen for hospitals, Souveniers.
What kind of skills do you have? Are you resourceful? Is there some service you can perform? -Look under 'services' or some such thing.
Can you haul campers? I know you have to put up a bond or high ins. and I don't know about the age.
Some people drive vehicles to a destination and return via public trans. or they've towed the spare vehicle.
Can you drive a dump truck? If you can do anything right now that would give you experience in something bigger than a pickup, you will have an edge when it comes to OTR.
For now, if none of these ideas work out, keep exploring options, keep your record clean, and just get used to someone telling you what to do. Everyone on the road has someone telling them what to do. (It's been so long since I've only had One boss. So much simpler then.)
I heard that "Two Men and a Truck" started out with 2 guys getting odd jobs using their truck. Would do anything that needed to be done. Now they have a business. It's probably also patented.
The thread of my thoughts here are: 1. take orders and advice well. 2. Be creative with your ideas and hop all over the computer to find new ones. Maybe you will find your niche doing something else. 3. Your record is the most important thing you can bring to this business. I want to be able to trust every driver out there, even though I know I can't, but at least it's comforting to know that most commercial drivers try hard to be careful. And, that most of them can't get driving jobs any easier than I could.
Let us know how you're doing. Maybe over time someone on here can help you get started. Lots of luck.