First of all draw a circle.
Now draw these lines it'll help you to locate the area to draw eyes, nose, ear and mouth.
Now start drawing luffy's face beneath the circle.
Now draw his neck by drawing these two lines.
Now draw his rest of the body and clothes. He's wearing a red shirt with open buttons.
Now draw his chest and this cross looking scar under his shirt as you can see in the picture above.
Now draw Luffy's eyes. He has big and round eyes. Also draw this scar beneath his left eye.
Now draw his nose and mouth. Draw his mouth at the end of the circle by drawing a line which is curved upward from both sides.