One thing you are bound to do right away is resize your image. If you do, be sure to first do the "Save As" so you have an original, source file on hand. Then do your resizing and choose "Save As" again, this time renaming the file to indicate that it is a smaller or larger version.
Now that we know the importance of saving, letís learn about resizing the image - in other words, changing the image's size without changing the pixels.
This may sound confusing at first but itís actually simple. You can change the image's resolution (i.e. from 300 pixels per inch to 72 pixels per inch), for instance, without changing the file size. In this case, you would not be changing your file; you would just be shuffling things around within your file. You would be causing your image to spread and cover much more real estate.
You'll likely be resizing the image often, because sometimes you want to print the image and other times you want to email it or display it on the Web.
In Photoshop, you go about resizing the image by using the Image Size command. Select this option from the Image menu, and a dialog box will pop up where you can change anything you want about the image's size and resolution.
If you need to make the file size smaller (say, to be emailed to a friend), then you will likely need to check the "Resample" checkbox. With this checked, you can reduce the overall pixel dimensions to an email-friendly size, such as 400 pixels wide. If you do this, be sure to use that "Save As" option right away, so you can save it with a different name and thus avoid overwriting your larger source file.