How to add index in database?

Hi , I want to add a new field 'sex' to an exiting table which has 12 fields already, then I add an index(username,sex) to use for query which find user's gender. (username is the primary key ) .Is it better or not compared to creating a new table which has only 2 fields username and sex ?

Yeah, sounds like you just need to add an extra field to the existing table, theres nothing wrong with having more than 12 fields.