I think the only real answer is "both".
First you need to read and learn and study everything you can about the issue. Make sure your challenge your own beliefs, and make sure you do everything you can to confirm that you are correct, keeping in mind that the worst possible outcome would be for you to accidentally become an activist for the wrong side of an issue (and that even being an activist that's right 70% of the time but wrong 30% of the time is a great deal of harm.)
Once you really understand it, then you need to become an activist on the issue. Learning is always helpful, but at some point you have to take that knowledge and use it to influence the world, or else it's not going to do much good. Take that knowledge you've learned and use it to educate other people, to communicate to politicians, to raise the stature of the issue, ect.
Fundamentally, a lot of political issues come down to helping people understand why X is better then Y for most of them and for the country or the town or the species as a whole. You need to have a significant amount of understanding yourself first, or else there's no point and you're not adding anything but randomness to the system; once you do have a significant amount of understanding, you have to take that knowledge and act, or else it's not doing anyone any good. That doesn't mean you stop learning; you always have to do both.
Suppose a high school student is deeply interested in a particular social or political issue and interested in doing something about the subject. What advice would you give him/her?
You may assume that the high school student is roughly similar in profile to the typical active LessWrong participant when he/she was in high school.
Some candidate social and political issues are listed below.
Topics of particular interest to the LessWrong audience:
Others that are more commonly perceived as important issues in the world today:
I asked a few people to opine on the last two issues on the list (in Facebook posts linked above) and the respondents generally focused on the acquisition of background knowledge rather than direct activism (i.e., read and learn rather than proselytize). Prima facie, this seems like sound advice. But it's quite possible that the set of people I interact with on Facebook is biased in favor of armchair stuff to the exclusion of activism. What do people here think?
Feel free to pick on one particular item in one of the above lists, or something not on either of the lists, and provide a specific answer for that. Or, provide general guidelines. Also feel free to specify additional assumptions (such as the country the student is in, or the student's other abilities or interests) and answer within the constraints of those assumptions.
PS: For some of the issues, you may feel that the issue is overrated or misguided (for instance, you may think that global warming is a non-issue, or that the status quo is optimal with respect to civil liberties or migration policy). In this case, your advice to the hypothetical high school student might largely be directed at making him/her come around to your point of view of the irrelevance of the issue. Comments suggesting you'd give advice of that sort are also welcome. If you'd simply suggest to the high school student to refrain from thinking about socially or politically charged topics entirely, that would also be interesting to know (cf. politics is the mindkiller).