Aha!  This is fairly easy.  You want the cropping tool.  In the toolbar,
it looks like a box with a diagonal line through it.  Or goto Image |
Crop.  It's close to what you've used.

You should get a new info bar at the top of the screen that asks for
dimension (pick one or enter your own) and the dpi (if you want PS to do
the resampling / interpolation - I generally do, to 300dpi).

Drag on the screen to create a box (which will keep the proper ratio
based on the print size entered); later, grab an edge of the box to
resize it (the ratio is maintained).  Grab in the middle of the box and
drag it around for placement, or do small nudges using the keyboard
arrows.  Click the green checkmark in the info bar (or lower right
corner of the crop) to commit the change.  Save as a JPG with highest
quality and you have a file pre-formatted for a specific print size.

It sounds like a lot of steps, but it's really quick.

Above is specific to PS Elements, which I'm told is a lightweight CS, so
I'd expect the same steps and menu options to be there.

Good luck, and have fun!

Cheers,
Richard

BTW, if you're really picky about the actual size, be aware that they
will probably enlarge the image a tiny bit to prevent getting a white
strip along the edge because of equipment mis-alignment.

At 4x6" prints, I've found this is about 4% (which means the image is
shaved slightly on the edge).  If this worries you, you'll need to test
with your print lab and compensate by sending a slightly larger image
than you want printed.

Cheers,
Richard