If you're a Class 12 student planning to apply for an undergraduate program in biology in the USA, it's generally advisable to start the application process during your senior year, typically in the fall or early winter. Key steps include researching universities and programs, preparing for standardized tests (like the SAT or ACT), gathering transcripts and recommendation letters, and working on your personal statement. Specific deadlines vary among institutions, but many have application deadlines in the early months of the year, so starting your preparations well in advance will ensure a smoother application process.