The OpenMRS Sonarqube console can be found on https://ci.openmrs.org/sonar/drilldown/measures/1865?metric=critical_violations
First, go to this page, and identify three critical issues that you want to fix. Note these issues as a comment on this page. Please go through the comments to ensure that the issues you picked out have not already been selected by other students.
Then fix these issues, and make a pull request to the OpenMRS gci2015 branch, which you can find at https://github.com/openmrs/openmrs-core/tree/gci-2015