You gotta learn to walk before you can run. Even freshman calculus (often Math 150) is difficult, and college math instructors tend to have assumptions that their students have a solid understanding of algebra, trigonometry, and precalculus. Calc professors tend to jump in feet-first right away and don't spend much - if any - time reviewing what they view to be prerequisites for learning calc. They will often flat-out tell students that if they fall behind early on, that they're better off just dropping the class and take a more basic math course first.
My recommendation to juniors and seniors in high school who are interested in a major where calculus is either a requirement, or something highly recommended, to take all the algebra classes, trigonometry, and precalc (if not full-on calc) before graduating HS. It's much better to learn it in high school because like most anything else, high school classes are easier than their college counterparts. High school calc teachers don't make the same assumptions about their students' prior knowledge like college instructors do, and a fair portion of the class will be review to get everyone up to speed before they dive in to the heavy-duty stuff. Kids do get a smattering of derivatives in advanced algebra and trig, but a week's worth of material isn't nearly enough to prepare a student if they're interested in becoming an engineer. If their high school doesn't offer anything beyond say advanced algebra/geometry/trig, their math teacher himself/herself may be a great resource as they likely do have some sort of a background with calculus, but if they don't (or don't feel comfortable with it), they most likely can make some recommendations as to how to best familiarize oneself with it. Perhaps a summer course at a larger high school, or some online materials would be helpful.