For anybody who actually bothers to sit down and try, it doesn't take much intellectual fortitude to understand the following:
- all else being equal, a larger aperture opening lets in more light and makes pictures brighter
- all else being equal, increasing the length of time the shutter is open lets in more light and makes pictures brighter
- increasing the light sensitivity of the sensor (ISO) makes pictures brighter for a given amount of light
I think I had these concepts sink in just fine after my first 5-10 minutes of reading on the internet. But, while understanding in a general sense what makes pictures brighter or darker is important, it's really just the tip of the iceberg. Before you'll be able to leverage your new understanding of aperture, shutter speed, and ISO to any extent at all, you still need to learn how the camera settings play off of each other, what depth of field is and how it's controlled, how motion capture is controlled, how camera shake is managed, etc., etc. and most of those things are not in any way "obvious" like the three points above; they only "sink in" through repetition and experience (i.e., practice, practice, practice).
That's why I think trying to jump right to full manual is a bit much, because unless you're capable of drinking from a firehose, you're probably going to feel overwhelmed by all the things you need to keep track of, which might be discouraging to somebody accustomed to snapping pictures with an iPhone. Start with the semi-auto modes like aperture priority and shutter priority, to help gain an understanding of how those parameters affect not only your results, but how they affect the other parameters.