Photoshop is basically made up of four areas: the menu bar, at the top, the toolbar just below it, the toolbox on the left and the palettes on the right. The menu bar and toolbox always stay the same, as they contain the different modes and options that you can choose, but the toolbar changes depending on context. The palettes are there to show the current status of your image, including the history of all the actions you have used and a thumbnail overview of how the 'big picture' currently looks.
To demonstrate the way the interface changes as you use it, try selecting the type tool from the toolbox (the one that looks like a capital T). You will see straight away that the toolbar changes entirely to allow you to set font name, font size and so on. In the history palette, your use of the type tool will be added to your history, and a new layer will be created for your text and shown in the layers palette.
Whatever you're trying to do in Photoshop, then, the chances are that your starting point will be either the toolbox or the menus. While the toolbox contains everyday tools such as selecting, filling and making shapes, the menus have more complicated functions like blurring, sharpening, and all the other effects Photoshop can achieve (mostly to be found under the Filter menu). When you have selected a tool from the toolbox, you can alter its settings using the toolbar – options from the menu will generally open a dialog box. Finally, when you want to go back and alter something that you already put on the image, you can use the palettes, although they have some other uses too, notably changing colors.