Rudimentary AI Enemies

I want to know how to make basic AI that will move around until the player gets in range and then it moves around and shoots. How do I go about that?

start easy with a rotation

  var position = this.entity.getPosition().x;
    this.position = this.entity.getPosition().x;
    this.position = this.entity.getPosition().z;
     if (((position <= this.X) && (this.Velocity < 0)) ||
                ((position >= this.Y) && (this.Velocity > 0))) {
                this.Velocity = -this.Velocity; 
            } 
this.entity.translate(this.Velocity * dt, 0, 0);

This is from my game the enemy rotates back and forth from there you can rotatye him other ways you will need this too

Enemy.attributes.add('X', {
    type: 'number',
    default: -1
});
Enemy.attributes.add('Y', {
    type:'number',
    default: 1
});
Enemy.attributes.add('Z', {
    type: 'number',
    default: 1
});
Enemy.attributes.add('Velocity', {
    type:'number',
    default: 1
});