Take the placement test at the beginning of the game. How well you do will affect what lessons you start on.
Clear levels and unlock more lessons by earning mastery points. These can be earned by succeeding in any of the mini-games.
Use the microphone that is built-in to the Nintendo DS to compare your pronunciation to that of native Japanese speakers.
Practice drawing hiragana to further your understanding of the Japanese written language.
Quiz yourself often to monitor your success. Consult the in-game dictionary and phrasebook often to build your vocabulary.