Use the Photoshop menu to get at the Preferences panel.

Use the File menu to open and save files and to perform Automated Batch Operations on files.

Use the Edit menu to cut, copy, paste, and transform the selected areas of your image.

The Copy Merged command is particularly useful. Instead of just copying from the active layer (like Copy does) it copies from all visible layers.

This is also where to go to change the Color Profile of your image. Color profiles are specific to your input devices (cameras & scanners) and output devices (printer) and if Photoshop knows the profile it can give you a preview of how the colors will look on that device.

Use the Image menu to change the size and overall color of the active layer.

Use the Mode submenu to switch between RGB (web) and CMYK (print) color spaces.

Use the Adjustments submenu to change the brightness, contrast, color balance, etc. of your images. Note that most of the commands in this submenu are also available as Adjustment Layers in the Layer Window, and that is a much more powerful way to use them.

Use the Layer menu to work with the layers that make up your document.

Or don't — I find that I can get at all the layer commands much more conveniently through the Layer Window.

Use the Select menu to create and modify your selection. Most software has only one selection command — Select All — but Photoshop has over a dozen different ways to select.

This is one of the most important menus in Photoshop, and mastering the fine art of selection is the key to success with Photoshop.

To learn more, see Selecting in Photoshop.

Use the Filter menu to apply standard operations to the active layer.

Note that there are a lot of filters, and the best way to learn about them is to try them out.

Use the View menu to control the way the document looks onscreen. The proofing tools are especially important if you use your images in print.

Use the Window menu to get at Photoshop's many windows, and to get at your own document windows.

Learn More

Use the Help menu to get instructions on how to use Photoshop.

Yes, I really mean it! The good folks at Adobe have put a lot of effort into creating good online documentation, and you can learn a lot here.