Celeste K.’s Answer
If you ultimately decide that you want to aim for a job at a big firm, you'll need to plan to attend an undergraduate school that will allow you to explore curriculum that excites you, but will also allow you to get good grades (3.5+ GPA) and get into good study habits for the LSAT. Some law schools no longer require the LSAT as part of admissions, but you will still need to take the GRE. Truthfully, your major matters less than your critical thinking, analysis, and writing skills when evaluating what you should study if you plan to get into law school and become a lawyer.
With respect to when your career starts after law school - it actually starts DURING law school. Many firms and organizations recruit on campus as early as your 1L (first year) year. Many career services offices do an excellent job preparing students for the process, but it is always helpful to set up individual appointments to understand the different firms and get further insight on when networking opportunities are offered. More law students participate in a summer internship between their 2L and 3L years - these, usually, 10 week programs are a mix of real legal work and social events to better understand the firm's culture. Most firms try to extend full time offers to their interns following the conclusion of the program. Grades should always be top of mind before obtaining an internship and you should keep your grades up following that as well.
Additionally, most students will sit for the bar exam directly following law school (I believe the CA administration is at the end of July). If you have an offer from a firm or other large organization, they will usually pay for bar exam prep and your fees associated with the exam, but will usually only pay for this once, and no one WANTS to take the bar more than once.