You must hold a B.A. degree before the beginning date of the grant. You must have sufficient proficiency in the written and spoken language of the host country to communicate and to carry out the proposed study. The earlier you start the better in writing a Fulbright application. Think about what really interests you or excites you. Applicants with a passionate interest in a subject find it easier to write a proposal.
Some countries have a language proficiency requirement, so be realistic.
If you don't speak Siswati, then maybe research in Swaziland is not right for you. Some countries are also much more popular than others and therefore are more competitive. Take this into consideration when designing your study or research project. You should begin the application process in your Junior year by narrowing the focus of your study to a project that can be completed in one year. In one to two pages, the project statement should give the what, why, how, how long, and where of your specific project and should give the reader a succinct picture of what you want to accomplish.