Well you COULD always use the pen tool if your program supports it, but for drawing freehand it comes down to 1) practice and 2) knowledge of how the material works.
the first is self-explanatory, like Self-epidemic said it comes down to confidence with strokes.
The second is your knowledge about how to define and picture a specific material. For example, hair is a lot flimsier than most materials, so you will have to know how to make the hair look flimsier than the flesh and skin of your character, but less flimsy than, say, water. It comes down to real-life drawing a lot of the time, but when that is nailed, I don't think that accurate strokes will matter as much. I'm not so good at drawing I can say it for certain, but I have a feeling that those two are complementary skills
It is just HOW you are drawing your lines, you're either being super nervous and thinking OMG I NEED THIS PERFECT or doing small, short strokes. You need to draw more freely, relax, draw long,swift lines rather than short ones. The more confidence you have the better your lines will become.
It is quite all right I would suggest looking at some videos of your favorite artists, you'll see that they draw lines very confidently, its something that comes with practice! Make sure you have your sketch perfect so that you don't have to worry that it makes no sense, that will help