How do I draw a curved line?
(Note that you can hand draw lines with the paintbrush or pencil tools. The steps that follow are for creating perfect, geometric curves.)

1. Make a new layer.
2. Choose the elliptical marquee tool. It may be hidden under the rectangular marquee tool in the upper left corner of the toolbox.
3. Drag a marquee from which one edge will match the curve that you want to make. Don’t worry about the rest of the ellipse.
4. Note that you can reposition your marquee while you are dragging it by pressing the spacebar while still holding down the left mouse button.
5. When you have an ellipse which matches the curve you want, release the left mouse button. To reposition a selection outline after you’ve drawn it, place your cursor within the selection outline (while a selection tool is still selected in the toolbox) and drag. You can also use the arrow keys to nudge it one pixel at a time.
6. With the elliptical selection active, and the new, empty layer selected in the Layers palette choose Edit > Stroke. Choose Foreground from the dialog box (you’ll need to make sure your current foreground color is the color that you want on your line before choosing Edit > Stroke). Click OK to add your stroke, and then press Ctrl-D, or choose Select > Deselect to deselect the marquee outline.
7. Choose the Eraser tool in the toolbox. Set its Mode to Block in its options bar. Or, if you use another mode such as Pencil, make sure its opacity is set to 100 %.
8. Erase the parts of the ellipse that you don’t want, leaving your curved line.