The coursework for STEM majors is notoriously rigorous. We don't say this to discourage students or force them to second guess their plans. Many interested candidates are more than capable. However, it does mean that it's critical to take a math and science heavy curriculum in high school (if possible). After all, the more preparation and training students have the better.
In other words, students leaning toward STEM disciplines shouldn't stop taking science and math classes even if they have met the minimum requirements for graduation. Further, instead of rounding out their schedules with a study hall, these individuals should think about enrolling in electives aligned with these fields. This could mean taking a computer science or environmental science course. Or perhaps following up a regular biology class with AP bio. Ultimately, students want to be able to demonstrate both continued interest and continued success in these subjects. This will certainly help strengthen their candidacies when it comes time to apply to colleges.
Additionally, students declaring a STEM related major on their college applications or applying directly to schools of engineering (for example), can expect that heavy consideration will likely be given to the scores they received on the math and science portions of the SAT or ACT. In fact, many programs maintain specific minimums. Therefore, it's critical to log those test prep hours to ensure they're ready.
By their nature, STEM courses can be rather demanding. In order to succeed, students should engage with these subjects as much as possible, in and out of the classroom. If they develop a passion for the subject matter through experience and exposure, they'll be able to enter college fully confident in their abilities.