You must complete the FAFSA (and sometimes the CSS Profile) during your senior year to calculate your official EFC and apply for financial aid. However, you can use EFC calculators to get an early estimate of your EFC to help you plan. To get an estimate of your EFC complete the Federal Student Aid Estimator or the College Board's EFC Calculator.
Note: The College Board gives you the option of using two different methodologies (formulas): federal, which is what the FAFSA uses, and institutional, which is what colleges who use the CSS Profile utilize.
ENGLISH Federal Student Aid Estimator
More info about the College Board calculator here.