Make a class, called something like movingText.  The class will have 3 member variables; x,y for position and a String to hold the actual text.

See: http://processing.org/reference/class.html

Code:

class movingText { 
  float x, y; 
  String s;
  
  movingText (float _x, float _y, String _text) {  
   //This function is the 'constructor'. 
   //It will get called when you create a new
   //movingText object
    x = _x;   //Set x to the first argument
    y = _y;   //Set y to the second argument
    s = _text; //Set our string to the 3rd argument

  } 
  void drawText() { 
    //Place code to actually draw the text here.    
    // see : http://processing.org/reference/text_.html

  } 
  // Now, two methods to change the x&y variables
  void move_y(float amount){
   y+=amount;
  }
  
  void move_x(float amount){
   x+=amount;
  }
}
 

Then you need the main program structure:
Code:

movingText myText;  //Create an object "myText" using the new class

void setup(){
 size(500,500,P3D);
  myText=new movingText(250,250,"processing rules"); //call the constructor
}

void draw(){
 myText.drawText();  // draw the text on every frame

}


void keyPressed(){   

  //This function is called when any key is pressed
  // if A is pressed, call myText.moveY(-1); etc..

  // see: http://processing.org/reference/keyPressed_.html
  // and: http://processing.org/reference/key.html

}
 

You will need to create a font (using the Processing IDE) and add code for loading the font.  

Always use the reference section! : http://processing.org/reference/index_ext.html

The processing documentation is unbeatable, and loaded with examples.   Good luck!