Some important things to remember:
(1) do not pay for graduate school. Good graduate (not professional) programs offer full support for 5 years, though that may include a teaching requirement.
(2) in the sciences, you will be done with courses in 2 years, perhaps sooner. Most science graduate students make it out in less than 6 years.
(3) many graduate programs have a shortage of students, so you should be able to pick from several labs (you all do rotations the first and possibly second years to get a sense of what different labs/advisors are like). When ranking your choices for graduate school, try to find programs that have several faculty doing research that excites you. You don’t have to pick an advisor before doing a rotation with them, but you do want to pick places with multiple options.
(4) be aware that a PhD program is very different from undergraduate. As a graduate student in the sciences, your goal is to make discoveries. You may take some courses to help you learn how to make discoveries, but mostly it is apprenticeship. You will learn by doing experiments that fail. It is not uncommon for your thesis and publicationsto be based on your last 18 months of work. Making discoveries is hard and unpredictable, but most graduate students graduate.