Arrow keys or WASD. Down while in the air to ground pound, down while on ground to crouch.
Sadly, due to lack of motivation, outdated sprites that I couldn't replace, and bugs I could've easily fixed if I approached this differently, Super Mario Bros. for Scratch only reached version 0.9.4 with only 3 worlds finished. The other 3 worlds are largely unfinished. Plans for this game will be abandoned for a better game currently in development. If you managed to get 500 coins you could buy a DLC from Toad's store. I forced myself to get a game over screen finished before abandoning the project. I think that I was too ambitious for my first Mario game. The one in development will return to Super Mario Bros. for the NES. I may return to this in the future, but for the seeable future, I will not.