How to select all of a webpage:  Select the entire page by using the ctrl-A key combination.
You should then see the entire page highlighted.

How to select part of a webpage: Click at the beginning of the text you want to copy.  Press and hold down the SHIFT key, and then click at the end of the text you want to copy. Or you can hold down the left mouse button while dragging over the area to select.  You should then see your selection highlighted.

Whether you selected all or part of a webpage, you can now copy the selection to the Windows clipboard with the ctrl-C key combination.  Often there is an Edit|Copy selection from the top menu bar which will do the same thing.