Inform yourself in advance, preferably at least one and a half years before your planned study abroad, about the Erasmus exchange with European partner universities offered by your department. You may find these in the list of Erasmus partner universities.
The responsible Erasmus departmental coordinator can inform you about the application procedure and application deadlines for Erasmus exchange places. Usually there is only one application deadline per year in the following academic year (for the winter term and summer term).