Someone wrote me asking for advice for her son who would soon be entering college as an undergraduate to pursue a scientific career. This is how I responded:
My advice is largely the same regardless of the specific career goal.
1. Early in your career / life, you do not know where you might end up, so key is to keep options open.
2. In almost any career with a technical component, key skills include being able to read well, write well, speak publicly well, do math well, think clearly, communicate well with graphics, work with other people, finish projects, etc. In terms of knowledge, there should be a basic foundation in physics, chemistry, and biology. I would focus on getting this broad grounding rather than in developing specialized knowledge.
Key here is the ability to think clearly, have basic quantitative skills, be able to present your ideas well both in written and verbal form, and be able to bring to completion projects that you start. Almost everything else can be learned at a later date. Graduate school is really the place for specialization. Undergraduate education is the foundation.
3. People do best doing what they love. I would advise my son (and I have!) to pursue their dreams with passion. When I was in high school, I could get by being a little lazy because I was smart, and I thought talent could substitute for hard work. To be competitive at an international level, you have to be both talented and hardworking. Your competition will be both. Being slightly obsessive is a positive characteristic professionally.
4. I would show my son this graduation speech, because I think Tim Minchin pretty much nailed it with regard to advice for young people: http://www.timminchin.com/2013/09/25/occasional-address/