(2). One thing that is really important is that you need to keep your original file, so my advice is that if you're playing with Photoshop copy the files that you're going to use and manipulate and put the copies in a new folder.
(3). Having copied the files over to a new folder now click on the file you want to play around with and open it.
(4). Along the top bar on the top left hand side now click 'Image'. A drop-down box appears and about the 2nd option down is the option for 'Adjustments'.
(5). Click on 'Adjustments' and another box appears with several options that you need to explore.
(6). Have a look at the following adjustment tools...
(c). Color Balance
All of these tools affect the colour and the tonality of the images. In some instances where your images are either too dark or too light Levels, curves and brightness/contrast may help to make the images more acceptable.
Color balance and Hue/Saturation all affect the colours in your images and can be used to either dramatically change the colours or correct the colours if they're wrong in some way (Colour cast).
(7). If you like the affect that you're getting with any of these tools, you can save the result once you're happy with it. You should make a note of what it is you've done and how you've done or better still make screen grabs and paste these into a Word File with an explanation of how you've done it and what tools you've used. If you really like the affect that you have used (You should do this for several) go up to the top left hand corner again and click on 'File'.
(8). Select 'Save As' and give the file a new name otherwise you'll 'Over-write' the original file and have to re-copy it from your original file. You could name it using the technique that you've used.