The old cliche “I can’t draw a straight line” has always been a bit silly, because of course it implies drawing a straight line should be the easiest thing to do. In fact, drawing a perfectly straight line is not really a natural thing for the hand to do, and you’re better off getting a ruler to help you out. That’s what the artist does.
What you do want to do, as a beginning artist, is to train your hand to be responsive to your brain and eye. You should begin by doing simple warm-up exercises. This is like stretching and light calisthenics for the athlete. If you’re wondering why you can’t be a great artist overnight, it’s the same reason you can’t go from your couch to Olympic gold overnight. It takes skill, and skill takes training and time as well as talent.
Whether you draw digitally or on paper, you need to get used to moving your hand the right way. Don’t hold the pencil like you’re writing necessarily, but a bit more perpendicular to the surface. Don’t use a death grip near the point, but hold the barrel lightly and firmly.
And get a ruler.
Use the ruler to draw a few straight lines to copy. I’m not kidding, give yourself a straight line to look at. Otherwise, it’s too easy to bend and curve your hand. Look at what it is that you want to do. Then copy that line several times, the best you can. Don’t worry about getting it perfect. The angle doesn’t have to be just so, and it doesn’t matter if you get it just plain wrong. You may find yourself drawing that line slowly at first, sweating bullets at the attempt. That’s OK. Just keep working at getting a more natural feel. You want to do your bending and light calisthenics until it feels easy, natural. Then you’ll find yourself drawing the lines faster and better.
Do the same with circles. Use something round to make a real circle to look at, then copy it many times, without worrying about making it just right. As with the lines, you are not trying to make pretty lines for other people to look at. Nobody is interested in watching you stretch and jog in place. This is all training, getting your hand used to making all the sorts of drawing movements you’ll need to do later.
After the circles, try drawing other sorts of lines and shapes. Anything you can think of. Do this anytime, and don’t necessarily make it a ritual that requires you sitting down and doing it on Wednesday morning. Instead, keep a scratchpad of some sort, and doodle whenever your hand is free.