Step 1: Inputs and Outputs
Step 2: Putting Inputs and Outputs together (Wiring)
In the picture one you can see a lever connected to some redstone wiring. When you activate the lever the redstone is activated which powers the door. in picture two you can see a lever hooked up to Redstone wiring which goes to two pistons. When you activate the lever the redstone current flows directly into the top piston activating it, but it also powers the block it runs into which is why the bottom piston activates. This is very use full and it is why picture 3 works.