Depends on if you mean a College Seminary or a Major Seminary.
A College Seminary would have requirements equivalent to any college. This is where you attend to complete the requirements for entry into a Major Seminary.
Those being a Bachelors degree, a second language, 30 credit hours of philosophy, and 12 credit hours of theology.
This would be for the United States and most, if not all, dioceses.
Religious institutions may have different requirements. Some of them run their own unaccredited theologates.