Are you stepping on the clutch? Have you assigned controls to the clutch? And if you are using automatic, are you pressing the brake while switching gears?
● If you're using the car with an automatic transmission, press and hold the brake pedal, then switch to the "D" gear (use gear up\gear down keys to do it).
● If you're using the car with a manual transmission, press and hold the clutch pedal, switch to the "1-st" gear, then very slowly release the clutch pedal and carefully press the accelerator pedal simultaneously.