CORE CONCEPT - OBSERVATION 
CAREFUL OBSERVATION OF A SUBJECT IS ESSENTIAL TO DRAWING OR PAINTING IT. 
Often the best artists are found frequently glancing back and forth between the subject and the drawing surface. That's because the more you focus on the parts of the object you're drawing, the better your drawing will likely be. 
 
Note that I said 'parts'. Often people have difficulty perceiving an item in terms of anything other than a mental image of what it 'should' look like. But what an item ought to look like and what it actually does look like are usually two very different things. Breaking free of the old way of seeing is critical to moving beyond 'child like' drawing.
 
A capable artist will be able to think of a scene not as a 'tree' or 'bouquet of flowers' or 'human face' but as a mass of shapes with different shades of color. By breaking down the object into its component parts, we can recreate it believably. A question was once asked, "How do I become a good artist without practice?" Short answer is that you can't. Drawing, like most things, requires some practice and a lot of learning, to improve your skills. What, you may ask, do I do if the object I wish to draw is not in front of me? What if I am a fantasy or science fiction artist? Well, even imagined objects would usually obey some sort of physical laws. 
 
Find a bunch of references of things in the real world 'like' the thing you are trying to depict, and pull details from them. Do you want to create a fantasy creature? Does it have fur or scales or feathers or skin? What animals come closest in the real world to your imagined creature? Pull details and forms from various parts of reality to make your fiction feel credible. The same goes for machines. Think about the machine needs to do - how people will interact with it and how it will work. Or fantasy characters - what is the character's personal past? Their personality, experiences, and the cultural background they were raised in? Again, just try to give everything in your work a bit of wear and tear, a bit of detailing, something that makes it feel like a real thing. Also, be sure the lighting and shading gives your scene a sense of physical form, making it something that obeys the laws of nature even if it is nowhere near our reality in other ways. A bit of this worldbuilding will almost always improve your imagined scene. 
 
What if my art sucks? I don't have good skills or good tools. Why even try? Skills can be learned. Tools can be acquired. These aren't impossible obstacles to overcome. If you want to try your hand at art (as a career or a hobby) and it's something that you're excited about, then just go for it! There are plenty of artworks that turn out badly. Even if you had a budget in the tens of millions of dollars, your creative work could still be bad. There are plenty of bad movies after all. But you know what? I've made hundreds of artworks and many of them were mediocre or outright bad. But generally, the trend has been clear; on average my art has been getting steadily better. Yours can too. That takes time and practice, and it means giving yourself permission to fail now and then. It means you have to accept valid criticism, and learn from it. It means you need a fairly thick skin. Not everybody will like your work. You just have to accept that there's a chance of failure and take a measured risk. Start with small short-term projects and if they're turning out decently, gradually increase the scope and ambition of your work. What's the worst case scenario if you make a bad creative work? You used up some time, maybe a few bucks, and someone hated the result. What's the worst case if you made no creative work? Well, then nothing happens. But let's put it another way: If you try to do something creative, it may or may not succeed. There's a moderate chance of it turning out well. If you do nothing, you have no chance of succeeding. Your work definitely won't turn out well because it doesn't exist. Then again, if you keep trying at something and nothing is working, maybe it's time to move on and try something else.
The banner links at the bottom of each page include links to my own webpages, links to my storefronts/shops online, and occasional links to other [third-party] verifiably legitimate stores and services I've hand-picked, vetted, and used myself. 
That said, I'll openly disclose that when you buy something from another vendor using these links, for example, a link to Amazon.com, I will get a small fraction of the sale amount and will use that as a means of supporting and expanding the projects I'm working on here. These ads for other vendors, as well as - especially! - links to my own shops, are helping me keep these sites online and growing with new content on a more frequent basis.