Need Help with movement (I am new and trying to learn)

I have an idea for a game but dont know how to implement the right type of movement. Its a 3d game and want a snake (Not the animal,the retro game) type movement.
I basically need a way to move forward constantly and when pressing A or D need to turn 90 degrees while keep moving.

Also if you have an idea how to keep camera to follow player constantly please tell.