Shifting a big rig is the hardest part of using the controls. There are strict rules for double-clutching, and the shifting doesn't emulate what happens in a car. Also, the same shift gates are used for two sets of gears. Depress the clutch, which is heavy and will build up the quads on the left leg of anyone who drives one of these things on a regular basis. Push the gear shifter into 2nd gear.
First gear is an extra-low gear designed for use with only the heaviest of loads.
With finesse and practice, shifting into gear will become easy. For impatient me, it was not. Release the clutch without using the gas pedal. A large truck doesn't require the driver to slip the clutch out while giving gas simultaneously. The engine produces enough torque so that the truck will engage the gear and begin moving without any application of the gas pedal.