Now add a "Lathe" modifier to it and under the lathe modifier there is the "Axis" gizmo. Move it in the viewport till you get the shape shown. The lathe modifier basically takes the spline you drew and rotates it around an axis defined by the axis gizmo. Where you put the axis gizmo determines where the center of the shape is..
Draw another line coming from the center of the shape you just created as shown. Select the vertex shown in red.
The corner on that spline is too sharp so we are going to have to fillet it to round it off. With the vertex from the last picture selected, hit the "Fillet" button.
Now that the fillet is activated, drag the vertex in the viewport and it will split in two and there will be a nice curve instead of a sharp corner as shown.