Check out how to make a simple 2d platformer. This has been broken into parts leading up to putting it all together.
start
Learn to set up an oak scene with a single potentially movable character.
moving
Get your character to move left and right.
falling
Implement basic collision and make the character fall if they are not on solid ground.
jumping
Bind the ability to jump if we are on the ground.
correct jumping
Update ground detection to only be able to jump on the ground not when falling through ground.
complete
Put it all together!