1. Start with talking with your teacher - ask questions in class about what you don't understand, let them know what you are looking for help on, go on their websites for information/resources, ask them about how you can improve in class and on preparing for your assignments, and ask about intervention options.
2. Take advantage of their websites/resources, all intervention options, and all tutoring options - don't rule out online tutoring options.
3. Be willing to put in more time outside of class on reading, taking notes, and studying. Read the books and take notes on what you read - this is absolutely necessary for college - get used to doing it now.