Changing the layout of a document's text on the page involves several subtleties not often realised by the beginner. There are interactions between fundamental TeX constraints, constraints related to the design of LaTeX, and good typesetting and design practice, that mean that any change must be very carefully considered, both to ensure that it “works” and to ensure that the result is pleasing to the eye.
LaTeX's defaults sometimes seem excessively conservative, but there are sound reasons behind how Lamport designed the layouts themselves, whatever one may feel about his overall design. For example, the common request for “one-inch margins all round on A4 paper” is fine for 10- or 12-pitch typewriters, but not for 10pt (or even 11pt or 12pt) type because readers find such wide, dense, lines difficult to read. There should ideally be no more than 75 characters per line (though the constraints change for two-column text).
So Lamport's warning to beginners in his section on “Customizing the Style” - “don't do it” - should not lightly be ignored.
This set of FAQs recommends that you use a package to establish consistent settings of the parameters: the interrelationships are taken care of in the established packages, without you needing to think about them, but remember - the packages only provide consistent, working, mechanisms: they don't analyse the quality of what you propose to do.
The following answers deal with the ways one may choose to proceed:
There is a related question - how to change the layout temporarily - and there's an answer that covers that, too: