Your puzzles will flow from the mechanics of your game, so you should start by defining those mechanics. For example, can the player run and jump? I would assume yes, since that's pretty much the definition of a platform game, but you should list that concretely.
What other stuff can the player do? For example, can the player collect keys to open doors? Can the player attack enemies somehow? If so, how exactly do they attack? etc etc
Once you've listed out all the different mechanics in your game (incidentally, you will probably come up with more later so don't worry about this being a complete list, just a place to start from) you can start planning ways for the player to accomplish specific tasks by using those mechanics.
If this is your first platformer then it's probably best to stick with mechanics that you've seen before and know work, but many innovative games have been built pretty much by starting with a couple new mechanics and everything else flowed out from there. Braid is a great example of a 2D platformer where all the puzzles started from the time-rewind mechanic and branched out from there.