There are no special rules for a platformer vs. any other game in this respect. I suggest that you just start making your game. You'll make a lot of mistakes, but hacking the game together while staying conscious of what one can do better is how I learned to create clean, organized code.
Here are some patterns that you should be aware of when making your game:
Two or three games from now, you'll be applying the principles that I linked to, and you'll be organizing your code better as a result of seeing how it all ties together from experience.
Another important point is that many people developing a game in their spare time do not finish it. Hacking your platformer together at the cost of a less than perfect code base might actually allow you to create a finished product. Then you can clean up the engine in version 2.0.