There are several ways to do this, one is simply to draw rects depending on your health.
Let's say your health is represented by a variable health, and the max by maxhealth, you could do :
Code:
gr.setColor(Color.white);
gr.fillRect(x,y,maxhealth,10);
gr.setColor(Color.red);
gr.fillRect(x+1,y+1,health,8);
If you maxhealth were to change but you want to keep the same size you could do :
Code:
gr.setColor(Color.white);
gr.fillRect(x,y,HEALTHBARSIZE,10);
gr.setColor(Color.red);
gr.fillRect(x+1,y+1,HEALTHBARSIZE*health/maxhealth,8);
Pretty classic stuff, concerning tutorials, good luck, it's pretty hard to find (though this could easily be covered by google)