Junior year is probably the most important year for your application- it's the last full year admissions will see. So getting grades up, even a little bit, really helps show improvement. If you aren't already, get really involved with clubs, sports, music- whatever you are passionate about. The reason they look at that isn't because they think you'll be the next stand out trumpeter, or the star player on the soccer team, but to see that you can actively and effectively time manage.
Next, SAT/ACT is going to be huge. ND is known to weight the standardized tests more...