Courses on Khan Academy are always 100% free. Start practicing—and saving your progress—now: https://www.khanacademy.org/economics-finance-domain/core-finance/money-and-banking/currency-tutorial/v/currency-exchange-introduction

Introduction to how exchange rates can fluctuate
More free lessons at: http://www.khanacademy.org/video?v=itoNb1lb5hY