Here’s a little bit of “food for thought” … chew carefully and slowly.
First of all, restrain your expectations. Rome wasn’t built in a day. If you set unrealistic expectations for yourself … if you are not willing to begin with baby steps … then you are going to do nothing more than mightily (and unnecessarily!) frustrate yourself. Begin with a wide-open mind: emulate a sponge. (And, fair warning! It’s highly addictive! :yes: Even if you have no long-term intention of making money at it… and they do tell me that it’s a ferociously over-crowded and therefore cutthroat-competitive field now.)
It would be a terrible waste … and it has happened many times before … for you to burn-out. Fore-warned is fore-armed. The holes are out there; don’t fall in.
Now, as to your immediate question:
Special-effects of any sort are based on the idea of compositing, and compositing is based on the idea of masking so that whatever manipulation you do will be applied to certain areas of the image but not to others. For example, when you render something in Blender or any other package, the computer generates not only “Red, Green, Blue” color information, but also a fourth channel of information called “Alpha” which determines transparency. When you watch any “making of” video of a modern movie, you see the large “blue screen” (or “green screen”) in the background, whose very-specific color is used (in a process called chroma-key) to generate a mask.
We’re all very familiar by now with what can be done with Photoshop, and one of the key aspects of Photoshop is Layers. In fact this is another example of the self-same idea of, first, by some means isolating portions (or aspects) of the image, and second, compositing them together (and selectively manipulating them) to produce a whole.
I repeat: beyond the inevitable first throes of frustration (“a sip from a firehose,” “nothing rearranges your outlook quite like a cast-iron skillet at thirty miles an hour” …) you will find a fairly-irresistible and very pleasant addiction that can be more engaging and rewarding than any puzzle you have ever solved. “Welcome to our nightmare.”