How can I get into Yale, Columbia, Princeton, or Harvard?

You should have waited on your ACT until your 2nd semester of your junior year and retaking it may work against you. If you are looking Ivy then your scores need to be quite high, ACT at a minimum should be around 30, and SAT around 2100. In addition, your class rank needs to be at a minimum the upper 10th percentile. Depending on your class rank you might be borderline and when you take your SAT it will need to be around 2100 to make yourself competitive, otherwise it may be out of reach for you.