The Edit menu allows you to manipulate stuff in Photoshop, be it selections, layers or Photoshop settings.
If you're new to PS, you may find the Undo function a bit peculiar. Pressing CTRL + Z undoes the last step, but pressing it again redoes it!
To just keep going back, press ALT+CTRL+Z.
One of the most useful tools in Photoshop is Free Transform.
It helps you in rotating, scaling, moving and distorting your selections and layers.
Once you activate it (CTRL + T), Photoshop will change into Transform mode, which you can exit by pressing ESC or apply by pressing ENTER.
You can manipulate the layer by grabbing the sides of the transform box and pulling them. To quickly change the transform mode, simply press the right mouse button.
Remember that you can also move the pivot point in the center of the transform box to change its axis. Very useful!