One of the easiest ways is to splt a circle in two, move the bottom half over then join them.
This can also be done with the pen tool you would half 3 points a beginning, middle and end. The control handles should be vertical. Moving the handles away from the point increases the curve and moving it closer decreases the curve. The length of each handle should match the other points.
You can also use the arc tool, playing around with that tool can result in a sine wave by using the tool 2 to 4 times.