To get that sketchy effect on your drawings, your going to need to purchase a Drawing Tablet which has Pressure sensetivity levels.
Im assuming you know what a Drawing Tablet is, but if you dont its simple a mechanichal pen which interacts with a Mechanichal board when pushed against it, so say you went on a program that is compatible with Pressure levels such as Photoshop (Flash doesnt work Pressure levels from what i know.) then you selected the brush tool, when you use the Pen on the Board it will draw, the harder you press the darker the turnout of your drawn result.
I suggest you get a wacom Graphire, thats what i have anyway, but there are other popular oenes too, look into the Volit0.
www.wacom.com