Before you move off
Driving starts before the engine does. Walk round the vehicle, check the tyres and lights, and look underneath for leaks. A vehicle that is out of order must be repaired before it is driven, not driven carefully until it fails.
Seat belts are mandatory for those seated at the front. Put yours on before you start, not after you have moved.
Set up the cabin while you are stationary: seat position first, then the mirrors, then the steering. You should be able to reach the pedals fully without stretching, and see both edges of the road behind you.
Finally, know where you are going. Deciding a turn at the last moment is what produces the sudden lane change that the driver behind cannot predict.