A dark, heavy line might seem comforting at the beginning since it makes your work instantly visible. But a few minutes later, when your line is so dark that you cannot erase it, your eraser starts to smudge your sketch and each attempt to correct things ends up leaving a stain, you will know that the path to cleaner drawing actually started even before the line.
Think of the first pass as an outline, not a drawing. Take a pencil, or use an HB or a light fineliner to test a few strokes on a scrap piece of paper; how much force do you actually need to make a line visible? Draw a few straight lines in one stroke and then curved strokes; pay attention to how little force you need to draw both straight and curved strokes, try drawing with the entire arm, or just your elbow, or your wrist. It is not really important what the line looks like, but it is important to notice how you draw it, press, drag or glide the pencil, how much force is there in your shoulder, in your wrist, in your pencil, and how it varies.
It is common that people press more when they are not sure about what is going on. It is very clear especially with the drawings of simple objects, where there is a curve or a small detail, for example the edge of the cup, a leaf or the edge of a key. Do not be afraid, stop for a second and look at the subject again, is the curve actually curved? It is round? It is flat? It is a narrow? Is it a straight, narrow? And do not draw anything, but simply imagine what the line should look like. A lighter line gives you more chance to do that without committing too early.
An exercise you can try at your next drawing session would be to draw a simple object with only a few lines twice. The first time use lightly only your construction lines, for instance an oval for the top of a cup, a cylinder for the body of the cup or a loose box to draw something small, keep drawing the line moving lightly and try not to define one contour right away. And then use the lines only for the edges you want to keep the darkest. By making the second line a little bit darker, you feel that you have made a decision for that line rather than just letting it happen.
The more lines are dark, the more the whole sketch will look dark. However you can always ask yourself whether you are building your edge out of many little lines. It might be useful when you are still learning how to use a pencil and you are still not sure whether the line goes that way or that way; the short lines can help you. But too many short lines might make the edge look nervous, try to draw each stroke with more purpose. For example, draw a curve, imagine your pencil is somewhere to the left of your paper and place it there, see where it could end, and draw your line towards that place. It is never gonna end up exactly where you want it, but it will help you learn how to draw lines with your whole arm, rather than just your wrist, and it will help you understand where to place your pencil.
The other way of thinking is that the more you are pressing your pencil, the thicker it will get. However, there is no need for the contour line to be the darkest; some parts of the outline, for example where the edges overlap, where you have more darkness or where you are drawing the most obvious part of the object, can be a little bit darker and the light lines could just stay behind as a construction line. You can also draw them as a contour line and just make it lighter, but not all the time, so you can leave it as a construction line and your edge would become the darkest of all. This would give a simple line drawing an idea of light and shadows, without having to draw them, because the drawing would be clean, without a lot of dark shading or cross-hatching.
Before you finish your drawing session, pick up your drawing and just observe the whole page, is there any area where the pencil was too heavy, too early? Draw it in your mind or on the paper if you like it, just draw the outline of the same object but this time with lighter lines and try it on your own. Cleaner lines happen in the smallest decisions you make at the start of your drawing: less force in your pencil, more looking and less drawing before you draw, and you should also try to draw the final line only when you are more confident about its place.