When any vehicle won't go into drive it is a transmission problem.
Go to a truck driving school. You're not going to learn how to drive a Class 8 truck by reading about it on this website.
You go to an actual truck driving school.
Get a truck/SUV with 4x4 and go find some mud to drive through and get your truck dirty.
You go to a truck driving school and learn how to drive a truck.
Someone who knows how to drive a truck and is legally licensed to drive a truck has to train you, so that you can drive a truck (but you do also have to get a license).
Go and drive and get there
When you buy, you can drive the truck as much as you want. But, when you lease a truck, you are only allowed to drive up to a certain mileage, and if you go past that, it will really add up in cost.
Not necessarily, however you do need a commercial driver's license to drive a truck for commercial use and in order to get one, you might need to go to truck driving school. Also, often these schools provide job placement, helping you get a job.
As fast as the truck will go... 4 LOW is the limited one < 30mph.
No it will not. Sounds like the transmission has a major problem.
You are probably low on transmission fluids