This short exercise will introduce you to what some 3D modelers have called the most powerful tool available to modelers, the humble extrusion.
Select two vertices along the bottom side of the shape you made, as shown on the left-hand side of the preceding screenshot.
Tap the E key, and then move the mouse to create another face as shown on the right-hand side of the preceding screenshot.
We discovered that if you press the E key when you have two vertices selected in Edit Mode, then Blender will create a new face. This is called an extrusion. You built a face, and extruded another from it. This is often how modelers create objects, such as automobiles, which often have fluid lines and no definite edges for much of their surface. Making faces and extruding them, you can make just about anything.