Train your drivers using driver manager. Select one and check the "training" option. Better do not choose the "balanced" one. First of all a driver needs the "long distance" skill which unlocks further options (at least two, three levels). Then you should train two skills. The choice should depend on the driver's initial experience. If your driver has two level of "long distance" and one of "ADR" it is worth picking one more time "ADR" and then switch to another, e.g. "fragile cargo".
However, do not neglect other skills. Your driver should have at least one level in all four skills (except for "eco driving" and already mentioned "long distance"). Remember that your drivers are not that entrepreneurial as you are. They may not find an order and come back empty which costs - you have to pay for fuel and their time. One level of each skill and more levels of long distance should help them find any order (but it is not 100% sure). Additionally, the better they are, the more they earn.