The Genki series is a very popular Japanese textbook used by students and teachers around the world. It works well in the classroom as well as for individual study (although, the individual student will want the sold-separately Genki Answer Key also)
For the self-study student, we recommend getting the workbook if you have someone who can check your work and penmanship. Otherwise, you might want to just start with the textbook and answer key.
Genki gives students a solid grounding in all four language skills: listening; speaking; reading; and writing