Outside lines are done a little heavier than the insides you can see Im using a 4 pixel brush at 100 % opacity
Now for the illustrating part. If you want clean illustrations you every time you stroke with a brush you must have a selection of the area preventing you from going outside. I persenally like to keep things clean so you can use the path tool and then convert it to selection and then proceed with brushing. Use a soft brush pretty big 50-60 pixels 20-30% opacity set to color black and stroke the sides. to get a nice line click once hold shift and click on the direction you want the brush to stroke. You can do you by mouse or if you are using a tablet this will make things a lot easier.