Colour Adjustment - you need to open the image you need to correct, go into the menu go to Image>adjustments>levels. There you will see a dialog box displaying a diagram of the colours in your image. Which can be seen in the image above this text. The black triangle is for shadows, the grey is for midtones and the white is for highlights. In the channels drop down menu you can choose beetween RGB. These indicate wether your changes effect all of the colours or just one.
Adding Flash - when you take a photograph indoors without a flash they will turn out under exposed and dark. To correct this you need to open the image you need to fix. In the main menu go to Layers>Duplicate. In the next window name the layer, Layer 1. Make sure Layer 1 is selected in the layers pallete. Select image from the menu, select adjustments and select exposure then you adjust the exposure and your whole image will lighten.
Cropping with the marquee tool - first you open the image you want to crop. Select the rectangular marquee tool from the tool box. Click and drag the mouse over your image where you want it to be cropped. In the main menu go to image>crop the image will be imediatly cropped.