A semi-automatic transmission dirt bike is a good halfway point between a fully automatic transmission and a fully manual transmission. You get the control from shifting gears yourself, but you don't have to worry about the timing and other advanced skills that you need to drive a dirt bike with a clutch. Dirt bikes are different from cars in that the shifting is done by foot, not by hand. If you can get your head around this, you should be able to competently shift within a day of practice.
Turn the dirt bike on with your foot on the brake, which is usually on your right side.
Turn the accelerator, and give it a little bit of gas. You should be in first gear.
Press on the pedal on the front of your left side with your toe to shift into second gear. This should be almost immediate.
Let up on the gas, and press the paddle shifter again to shift into third gear.
Repeat the process to shift into fourth gear.
Press the pedal on your left side with your heel to shift back down. Shift to second gear for tight turns or if you need to dramatically slow down.