Never change lanes without giving the proper signal and looking to make sure the move can be made safely.
Here are the steps for making a lane change:
Check your mirrors for a space in traffic where you can enter safely.
Check your blind spot by looking over your shoulder in the direction of the lane change. Be especially careful to check for bicycles and other small vehicles. Signal that you want to move left or right.
Check again to make sure the way is clear and that no one is coming too fast from behind or from two lanes over on a multi-lane road.
Steer gradually into the new lane. Do not slow down, maintain the same speed or gently increase it.
Never make sudden lane changes by cutting in front of another vehicle, including bicycles. Other drivers expect you to stay in the lane you are already in. Even if you signal, they expect you to yield the right-of-way.