Constructing a figure-
Image 2- This is exactly the method I use to draw Sonic, Mega Man, the Ninja Turtles, Teen Titans, real life people, whatever character I'm going to put down. Discover what basic overall shapes make up the character so you can throw them down fast, giving yourself a base or a framework to build off of, and then use that base to ensure that you have a solid drawing before you go through the hassle of drawing details and "pretty" lines.
So many amateur artists, myself included, have wonky drawings because they didn't first ensure they had a solid base to draw on top of.
Like Mickey, Sonic's head is a circle first and foremost. Take a look at Uekawa's sketches and you'll see the circle and the cross marks to anchor Sonic's face still evident. I'm sure you can find plenty of tutorials for how to draw Sonic that shows you where to go once you've set your circle down. Be aware that you aren't trying to learn "how to draw Sonic standing with legs crossed and waggling a finger", avoid examples where its step by step instructions of "first draw a wavy line, then two dots on top". You want to learn what shapes are used to build off the circle, how and where his ears line up on his head, how big his shoes are compared to his body and all that kind of stuff.
Find all the shapes that make Sonic as a rough frame work, and get used to drawing those shapes in different views for different poses. Build off those rough drawings, adding his details to better understand how to pose and "move" the character in space.
If you can master construction drawing you will master character drawing.
Perspective- Find tutorials/books on learning perspective, establishing vanishing points, and horizon lines.
I haven't read through these carefully, so these might suck, but at first glance they look helpful in teaching the fundamentals for establishing perspective, and most importantly, why and how you would want to use it!