The four main mathematical topics in Sixth grade (taken from the Common Core Standards) are:

(1) connecting ratio and rate to whole number multiplication and division and using concepts of ratio and rate to solve problems

(2) completing understanding of division of fractions and extending the notion of number to the system of rational numbers, which includes negative numbers

(3) writing, interpreting, and using expressions and equations

(4) developing understanding of statistical thinking

* These are general overviews.  For more specific standards, follow the "Common Core Learning Standards" link on the left.