When drawing it is crucial to appreciate the process rather than to focus on the end product. The business of looking and translating what you see into a two-dimensional image is absorbing and relaxing as well as demanding. The fact that pens can't be rubbed out emphasises the value of a bold and explorative approach.
There are many ways to use a pen and many marks to be made. A good starting point is to have a ‘play’ with a range of pens. I tend to use the Uni-ball Pin fine line and there is quite a difference between, for example, 0.03 which isfine and 0.8 which makes a much darker, thicker mark.