You may not take all this in at first. Make sure you understand the basic operation then use these notes to experiment with the details later.
Try not to have to enlarge your image (increase the number of pixels). You will always lose quality - though probably not as much as some would have you believe.
Use this if your picture is on its side or upside down.
On the menu bar select Image > Rotate and then the appropriate option.
Cropping allows you to select a rectangular part of the image, completely throwing away the edges (background) that you don't want. Once cropped you cannot get those parts back. (Moral - make a copy before you start).
Use the crop tool - two overlapping right angles in the tool bar on the left..
Click the top left corner of the area you want, hold the button down, drag the mouse to the bottom right corner and release the button. If you are satisfied, double click within the selected area or click the 'commit' button.
If it's completely wrong, hit escape or the 'cancel' button and try again.
If it's almost right adjust the selected area by putting the cursor on one of the side handles and dragging it in or out.
You can rotate the selected area by moving the cursor just outside one of the corners of the selected area. When the cursor changes (double ended curved arrow) move it around until you get what you want.
Control the crop by:
Shift Key as you drag - forces the result to be square
Alt Key as you drag - drags the selection from the centre
If you want the image to be a particular shape (e.g. to fit a photo frame) then set the aspect ratio or the width and height in the options bar at the top of the Photoshop screen. This will force the crop to the required height/width ratio. Leave the 'Resolution' blank for now.
You may want to make your image smaller if, for example, you have a large image for printing but want a smaller one for emailing. You may want to make it bigger if all you have is a small image and you want an enlargement.
All resizing, particularly enlarging, loses some quality. Therefore do it as little as possible. Don't resize something which has already been resized - it's better to go back to the original.
From the menu select Image > Image Size
Select the new size you want (work in pixels preferably)
In Photoshop you may need to select 'constrain proportions' and 'resample image'.
'Bicubic' will usually give the best results when enlarging photographic images. (When enlarging, Photoshop has to invent new pixels. Bicubic introduces some complicated maths to decide the best colour for these new pixels. 'Nearest neighbour' will just enlarge the existing pixels and edges will look jagged.)
With Photoshop you can crop and resize in one operation. Using the crop tool set the height and width in pixels in the options bar. Or you can set the height and width in cm (or inches) and the resolution. If you are intending to print the image set the resolution to 200 pixels per inch.
Think of the canvas as the piece of electronic paper on which your image lies. Normally it is the same size as the image. You may want to make it bigger, e.g. if you want a white border around the picture or to place two pictures side by side and treat them as a single image.
From the menu select Image > Resize > Canvas size. Adjust the height and width to the new values you want (they must be bigger then they were, otherwise it does not make sense). If you want your existing picture to be at one side or corner, rather than in the middle, select the position. You can choose the colour of the extra canvas in the box at the bottom of the window.
You can choose whether to work in pixels, cm or percentage of the existing image, and any of these can be absolute or relative to the existing image.