More importantly, do the following:
1. Do what interests you. If you think law's the direction you want to go in, your major really doesn't matter too much. Interesting majors = interesting courses = interested students = greater prof/student relationships = better LoRs and higher GPAs. But the last two shouldn't even matter.
2. Do NOT freak out about one bad grade. A bad grade is not the end of the world. Even two won't kill you. That said, do put effort into your work, but not for the grade, necessarily. Do it because you owe yourself the education.
3. For ECs in undergrad, do stuff that looks like fun. If you're not going to have fun doing it, why do it at all? Someone else would probably love to take your spot, and there's likely a cause just as worthy out there that requires your attention which you would love a lot more.
4. Take classes that force you to read and argue. The best ones in this vein would be history, philosophy, or literature courses. Philosophy is probably the outstanding candidate in that list. Learn to read critically and write lucidly.
5. Germane to the last point is my fifth commandment: LEARN HOW TO WRITE. Do yourself a favor and explore the essentials of essays. Learn what's right and what's wrong. Read Elements of Style by Strunk and White. Understand the concepts of thesis, antithesis and synthesis; experiment with flow. Most importantly, recognize that the essay you write in university is only one type of essay that's been privileged by academia, but in order to transcend this mental manipulation, you will have to master their "tools" first.
6. Finally, relax. Find ways to enjoy yourself. Sure, your future is important, but life isn't only about tomorrow. And, in the end, you will have other options. Don't let the potentiality of law school paralyze you.