Really good website to help you start planning: Café College - click on the sophomore link
Continue to take challenging courses
Keep your extracurricular activity worksheet up to date
Start thinking about what you want to do after graduation - college/military/trade school/work??
Pick a few colleges that you are interested in and find out the admission requirements for those schools
Join or continue participating in clubs and organizations
Look for community volunteer service
Talk to your counselor about your graduation plan
Monitor your grades and know your 4 year plan
Take the PSAT, if available
Update your resume