How to draw a wave in photoshop

Rather than drawing the wave from scratch you could do this --

From the Shape tools, select the Rectangle tool.

Make sure that Shape is selected in the Options bar.

Drag over an area to create a rectangle.

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Select the Pen tool.

Click the top edge of the rectangle in two different place.

That will add two anchor points.

Select the Direct Selection tool.

Press on a point and drag it into position.