Besides learning anatomy, You essentially want to deconstruct the figure into it's most basic shapes. Forget about the intracacies of a smiling face or the wrinkles on your fingers. You want to turn every body part into some sort of shape.
That muscular man is nothing more than rectangular prisms and cylinders. The same can be done for any complex machine or fantastic animal.
You can practice by finding pictures of athletes or any kind of pose and tracing over them with paper or on a digital software like Photoshop. Once you feel comfortable enough, you can sort of do it by eye just by looking at the reference.
If you get a grasp on that and then feel like maybe your figures are kind of lifeless then I would recommend incorperating Lines of Action into your figures.
I feel like this can be practiced by doing figure and contour studies of real life models or reference pictures of people. This allows you to find the motion a particular movement conveys.
It's always fine to take your time when starting out but this is something you don't want to spend too much time on. Try and find the lines quickly so that you have more time to draw the rest of the picture.
Things like expression and lighting and perspective are all really useful for conveying parts of a story like setting and tone but I believe they are the icing on top of a well made cake which is solid figure drawings.
One more very important thing; Don't marry your work. You should not be afraid to scrap an idea if it isn't working. Nobody is judging you while you draw so feel free to draw the shittiest stick figures before you're ready to move on to blocking them and adding facial expressions.
There are probably 101 things I'm missing but I've got things to do. I hope this helps a little bit I know I was really pleased to learn this kind of stuff.
The images I used are from Drawing Comics the Marvel Way, an older tutorial book that sort of runs down the pipeline of how to create their style of drawings from scratch. Feel free to not take everything they say as gospel but I feel like they are very comprehensive when it comes to drawing convincing and exciting figures. I would also reccomend Understanding Comics by Scott McCloud and even Making Comics by him if you can find it. It honestly changed how I looked at any piece of art, including comics. from that point on.
Happy drawing!