Frankly, the only things you really need to understand in PS for photography to start are:
- Open from Bridge, very easy to do, just go to a folder and double click on an image
- Color space selection, "color profile" in edit menu: just select ProRGB and check all the boxes asking whether you want to be asked what to do in case of inconsistences between files,
- Info palette: just go to window menu and display the info palette in which you can display the RGB value of a pixel or group of pixels
- foreground/background color: at the bottom of the left tool bar, you see 2 little squares overlapped. The one on top shows the current foreground color, and is the only one you need to worry about to start. You can click on it to change the color that will be used to paint with the brush tools (see below)
- History: just go to window menu and display the history palette which provide multiple undo in case you do something wrong
- Layers: just go to window menu and display the layer palette in which you can select which palette is the active one, you can show/no show layers or change their order. Think of old plastic transparencies stacked on top of each other. By default a layer is transparent and contains nothing,
- Resize: this is part of one of those menu a bit towards the left and enables you to change the number of pixels in your images or to change the size of the canvas,
- Brushes and cloning tool: in the left tool bar, enables you to paint or copy a part of an image on top of another (hint, you need to alt click somewhere in an image to select the source of the cloning operation),
- gradient tool: also accessed from left tool bar, is very useful to generate progressive applications of adjusement layers though masks (see below),
- Masks: can be created by clicking on an icon at the bottom of the layers palette (second from left): they make it possible to control what part of a layer impacts the layers below it. You can click on the small mask icon in the layer after having created one, then you can use the brush tool (see below) to paint on the layer,
- Adjustement layers: special layers that provide the ability to do live modification to an image. It is pretty intuitive and is the only good way to do images modifications in PS,
- Sharpening: Filter menu, then you have a few options to sharpen your images, just try.
Easy.

Cheers,
Bernard