Where do I Start?

Outside of school, most of your learning is going to be from studying (or even copying) examples and looking up documentation. Figure out what you want to make, break it up into pieces, then look for examples of how to make each of those.

Want to make a tetris clone? You need to be able to draw shapes, move them based on a tick, and clear lines. The first and second can be learned by googling around for a framework or library (JavaFX, Swing, LibGDX in Java), the third is a straightforward programming problem.

Thanks

The question you asked is almost as broad as "What's the meaning of life?" You could be more specific about your goals -- are you looking to work in 2D or 3D? What programming language do you prefer? What style of platformer are you looking to make? (Super Mario Bros is a platformer, but so is, e.g., Tomb Raider on PS1, or Super Meat Boy, or Rayman, or.... many others)...

But we're an ambitious community, so we're going to try to answer, anyway :-D

Archduke is right: if you're just getting started in game development, you're going to need to do a fair amount of "copying", or to be more precise, following along with tutorials, books, whatever you can find.

Some other points to consider:

• Video games are a somewhat specific type of computer application, with their own set of design patterns and "best practices". As you've probably already experienced, it can be tough to learn how to make a game end-to-end, because a lot of online tutorials focus on bits and pieces of the game, but not the entire thing. And, it can be tough to know which bits and pieces to learn.
• If you're trying not to use a ready-made engine, then you're going to need to write your own engine. Engine development involves learning some pretty general computer science and software design principles that are good to learn (personally, I enjoy the engine side of game dev more than the game dev side, heh)

I'm probably rambling incoherently, so I'll get to the good stuff. There is no shortage of resources you can find online to help you learn. Here are a few I could think of off the top of my head. Your mileage may vary:

https://katyscode.wordpress.com/2013/01/18/2d-platform-games-collision-detection-for-dummies/ (a blog series on making a platform game)

https://inventwithpython.com/chapters/ (A free book about making games, with Python - focusing more on program logic than graphics)

https://inventwithpython.com/pygame/chapters/ (A free book about making games, with Python/Pygame)

https://unity3d.com/learn/tutorials/topics/2d-game-creation/creating-basic-platformer-game (A Unity tutorial for making a platformer -- I know you said you don't want to use an engine, but.. you might consider it, because writing your own engine will seriously take much longer than using one that exists already, heh).

http://gameprogrammingpatterns.com/ (A reference book -- won't really teach you how to make games, per se, but will explain some things that are useful to know when making a game)

http://www.gamefromscratch.com/ (A simply awesome website)

...etc. There's a lot out of info out there.

Good luck!

Not a game design topic. Moving to For Beginners.

I was going to suggest you read the For Beginners FAQ, but with the site update over the weekend those seem to have vanished.