We require that you include all grades for coursework undertaken after the first two years of study (typically the last 60 semester units, or last 90 quarter units), not just grades for subjects taken in the major or grades which seem relevant to your proposed program of graduate study.
If you hold more than one basic degree, calculate the GPA on the most recent degree program.
If your GPA is not on a 4.0 scale there is no need to convert it. Enter it as is, in the appropriate GPA box on your online application.