How familiar are you with Rust and Bevy?
If you're learning Rust, I'd recommend actually starting with the Rust book:
https://doc.rust-lang.org/book/
If you're learning Bevy, I'd recommend going through the Bevy book and then checking out the Cheatbook and examples as needed:
https://bevyengine.org/learn/book/introduction/
https://bevy-cheatbook.github.io/
https://github.com/bevyengine/bevy/tree/latest/examples (note that this uses the latest branch instead of main)
Otherwise, where you start is really up to you 😄
For a 2D platformer, I'd probably start with just creating a basic flat stage so I can build out my player movement, then move on to stuff like bigger levels, then more gameplay-related stuff (enemies, score, powerups, etc.).