Get hold of at least two textbooks. In economics even more than elsewhere, each book will give you a different perspective. If you're feeling really adventurous, pick up a second-hand textbook that's a decade or two old, and keep an eye on how the orthodoxy has shifted. Which to buy? The best-sellers are all pretty similar: quite clearly written, but maybe longer than they need to be. Mankiw, Samuelson/Nordhaus and Baumol/Blinder all seemed OK to me; I'm not aware of any truly exceptional econ. textbook.
Also recommended in this reply