Do not start off drawing hard!
Start your sketches with light pencil markings and drawings so that it is easier to rub off mistakes and modify any changes. When shadows are added, the areas that are meant to be kept light will not need to be changed, only the dark areas will just need the graphite to be applied on harder.
Check your head proportions!
When drawing people, if the head size is wrong, everything else will go wrong and the body's proportions would be different and weird!
Don't use a ruler!
It is best to not use a ruler when you draw so that you learn how to not use a ruler to draw straight lines. To be able to draw straight lines without using a ruler is an artist's most important skill. You hands will also learn how to become steady and it will help you draw all sorts of line better.
Be confident in what you draw!
One of the most important things is that you are confident in what you draw and draw what you see. If you aren't confident, your brain will muddle up what you see and you would end up with a picture completely different from what you want it to be. Just be confident in what you draw and the result will come out pleasing to yourself.
There is a saying that "practise makes perfect". Though many would say perfection does not exist, if you draw as much as you can, your drawing will become more and more alike with your object and will even become realistic