Yea I don't get why everyone's being so mean to the OP, he's 19 years old. Also, he's right in that many people who go or who want to go (myself included) to law school do so because they are interested in and want to influence policy in some capacity.
To try and help you to clear some things up, there are many ways in which a JD can help you to influence policy. Being a judge, litigation for a PI/non-profit, government, working at a think tank(although this usually requires a degree in another field as well or if not then decent work experience.), etc. I think what other posters are trying to get at is that most of these fields involve actual lawyering, so if you don't understand and think you will enjoy the practice of law, then this may not be the best route. Especially if you want to do something like advocacy, where the JD is not necessary and you don't waste time and money studying law.
Quick aside, I saw a poster mention how you originally wrote about pursuing law if getting to the phd in Econ is too rigorous. From being in TLS and seeing other posters react to similar statements, I'm assuming that the poster was essentially mocking you for your naivity because they probably think that getting into law school is so rigorous in itself. Before anyone else tries to attack this part of the OPs post, I went to a top undergrad where one of my majors was economics and know people who went on to get top Econ PhDs...I would argue that it is objectively MUCH more difficult to get into a top 10 Econ PhD program than it is to get into a top 10 law school. Like it's not even close.
OP to answer your original question, I would personally wait a while. What the other posters are getting at is that, depending on your timeline, studying for the LSAT will be the equivalent of a part time job or 1 or 2 extra classes. Just due to time constraints, this can have a negative impact on your GPA. You can take the LSAT whenever you want, but you only have one shot at your undergrad GPA so you're better off focusing on maximizing your GPA. Also, dude you're in college, have fun. It's only 4 years. I get thinking ahead and all that, but I'm working full time now and trust me, it fucking sucks. I would do anything to be 19 and in college again. If you study for the lsat, combined with your class load, other ECs you're involved in, and preparing/interviewing for internships, you'll honestly have no fucking life. If I were going to take it in undergrad, I would wait until those times that I had no classes like in between semesters or internships and study like 40 hours a week during those periods. Either way, you're only 19. Take some time to do some research and see if law is really for you and enjoy your time because it only comes by once!