First, you should define a Displayable object and set it as current Display for your MIDlet, with the Display setCurrent() method.
Secondly, you should perform low level graphics paint operations within Canvas paint() method, whose argument is the Graphics instance you're trying to use in your code.
For example you can extend Canvas this way:
Code:
public class MyCanvas extends Canvas
{
protected void paint(Graphics g)
{
g.drawString("welcome to the Matrix", 10, 10, Graphics.BOTTOM | Graphics.RIGHT);
}
}
And then, in your MIDlet startApp() method, you can do:
Code:
Display.getDisplay(this).setCurrent(new MyCanvas());
Hope it helps 
Pit