Hi Grace,
(I might need a bit more information.)
R activates the Rotate Operation (unless you changed the Keymap in your Preferences) and pressing X after that makes it into a Rotation around the X-Axis and entering a number after that will tell Blender how many degrees it should Rotate; R, Z, 90 will Rotate the Selected Object 90° around the Z-Axis. (do not press Enter in between, only after you finished inputting.)
I recommend watching at least https://cgcookie.com/courses/blender-basics-an-introduction-to-blender-4-x before attempting this Course.
