Put the mouse at the beginning of what you want to copy and click the left mouse button, holding it down while you drag over the section.

Drag across or down. At the end of what you want copied, let go of the mouse button.
 

(If you goof, click the mouse button once to unhighlight, then start over.)

While it's highlighted, hold down the Ctrl key and hit the letter "c". This copies the highlighted part to an invisible clipboard.

Now go to where you want to paste it. Put the cursor on that spot by clicking the left mouse button once. Now hold down the Ctrl key and hit the letter "v" to paste what you copied.

If you want to move text from one place to another, highlight the text then hold down the Ctrl key and hit the letter "x". Click on the place you want to move it to, hold down the Ctrl key and hit the letter "v" to paste it in the new place.
(This cuts it from the original place, copies it to the invisible clipboard and then pastes it in the new place.)

HIGHLIGHTING USING THE KEYBOARD

Another quick tip to make life easier is learning how to use the keyboard to highlight things.

Put the cursor at the beginning of what you want to copy by clicking there once with the left mouse button.
Now hold down the Shift key and use either the arrow keys, the Home key or the End key to highlight.

The arrow keys are self-explanatory. (Try it!)

Holding down the Shift key and then hitting the Home key will highlight from the cursor to the beginning of a line.
Holding down the Shift key and then hitting the End key will highlight from the cursor to the end of a line.