the sections are averaged to give you an overall Academic Average and Total Science average. AA is reading, math, and bio,chem,orgo. TS is just bio/chem/orgo. national average is around an 18, but you want to shoot for 20+ on all sections. a good score would be AA and TS of 20+
if you want to get a feel for the sort of scores many accepted applicants have, you might want to check out predents.com