To find a career there several general guidelines to meet.
Do your best in school in terms of grades as well as extracurricular activities.
Build up your resume and have it reviewed for content as well as formatting by professionals that you trust. Having a professional looking resume is as important as having a content strong resume. If your school has a Career center service, or a similar type of student service for people graduating soon and looking for their first job, then take full advantage of that. Mock interviews to improve your skills and comfort will also help with landing you in your dream job.
Research jobs online from sites/boards such as Indeed.com
Here is an example for a search of Marine Biology. If you have a location you'd like to live or one that you've researched is good for the job market you're interested in then you can also search by that.
For a more targeted search you can speak with your teachers/academic advisors on the field as they will have first hand experience and advice for you.
Professional networking also helps with job searching. Consider creating a LinkedIn account which will help with making professional connections.
And lastly keep a positive attitude. Job searching can be stressful and takes different people different amounts of time. The more effort you put in the more it increases the chance of not only getting job offers, but finding your dream job.
Best of luck!
Teng Keng’s Answer
With a masters or Phd, you'll be able to apply for an ecologist/researcher position. Keep in mind, these positions are rare and a lot of people will be applying for them. The ecolog listserve and the Texas A&M job board are the gold standards in America for ecology positions. Marine biologist positions will be posted there as well. I'm not sure how useful they will be for someone not in America, but I occasionally see international job postings there as well.