basic photoshop tools

Basic tools:

Paintbrush (adjust size, opacity & hardness on menu toolbar)

Paintbucket (adjust opacity & tolerance)

Eraser (brush options just like paintbrush)

Color replacement brush (set the color in the foreground color picker)

Inverse selection

Use basic Image-Adjustment options to fine tune your images before you get started:levels, brightness/contrast(use legacy for more dramatic effects) color balance, hue & saturation
Use selection tools to grab areas - either to copy/paste or to manipulate color or light levels:marquee rectangle, lasso (polygonal & magnetic; delete to back up an anchor point at a time), magic wand(quick select is a mixed blessing) change the feathering options or refine edges afterwards for a smoother look
You can add to or subtractfrom a selection as well; hold shift & select to add more, hold option & select to remove an area. Remember that you can save a selection (select menu, save) and reopen it at a later date without losing any work.
Each layer has its own opacity & blending mode options on the layer palette.

For now, clean up 3 photos with the color & level options - one of each genre; landscape, portrait & abstraction. Then, create a new file and make 2 diptychs where the combination of 2 photo files makes a new relationship. Start by creating a new file, 8x10" at 300dpi (make 300 your default resolution), create a new layer (on layers palette) and use the selection marquee to draw out a border; fill it with the paintbucket tool.