So you want to be an economist? It's not an easy job, and especially after the recent financial turmoil, it has not been the most desirable job. However, the steps you need to take are quite simple.
High school: Nothing special to be done here. There is nothing you can study in high school that will especially prepare you to become an economist. Try to focus on math, though, and pay attention in English class. Economics depends on both of these skills to excel. There is probably a high school macro or micro economics class you can take, but remember that its concepts will be very fundamental and will simply be repeated in your introductory university courses.
College: In your first year, take an introductory macro or micro class. This will tell you whether or not you will enjoy economics, and whether you can handle the figures, statistics, theories, charts, graphs, and lectures. Low level economics can be a real slog, but it's something every potential economist must go through to make it to the 'fun stuff.' If you find you can make it through foundational economics, then congratulations, you have what it takes to be an economist! Declare your major and look into higher level economics courses. As an undergraduate economics student, two things are very important: learning high level math, and doing research. The math part is simple, assuming you are competent at math: just take math courses. As far as research goes, you have to do everything you can to get taken under the wing of a professor. In economics, research experience is king, and having it will give you a significant leg up on your peers.
Being an economist requires a minimum of a four year college degree, though it doesn't necessarily have to be in economics. Once you graduate, you have two choices: either go to graduate school, or enter the job market. Either is a fine choice on the path to becoming an economist, but remember that if you want to teach economics or work in high levels of government, you will almost certainly need to attain a graduate degree, and most likely a doctorate.
Once you have a job in economics, or you're in grad school, you're an economist! Simple, huh? Well, not really. You need to be doing serious, high level, original research and contributing to the field to be considered a 'real' economist. It takes a lot of hard work and dedication, which is a lot to ask of most people. Economists are a rare breed.