It's mostly up to you. The standard advice is "upvote things you want to see more of, downvote things you want to see less of", and that's not a bad starting point especially on the upvote side, but there's a little more to it than that.
People respond more strongly to downvotes than upvotes, so most people don't downvote merely low-quality posts even if they'd like to see less of them; that takes something more severe. I tend to downvote stuff if it seems to have been posted in bad faith, or if I find it rude, unreadable, obnoxious, or excessively partisan; I'll also sometimes downvote top-level posts (not comments) if they're poorly written or uninformative, especially in Main.
Downvoting posts because you don't like the people that wrote them is explicitly against policy. Don't do that. On the other hand, you'll probably find yourself downvoting the same people a lot, and that's fine as long as you're basing your judgement on the posts themselves.
Incentives-wise, there's one major caveat, and that's that comments (but not top-level posts) downvoted below -4 can no longer be responded to without paying karma of your own, and neither can any of their children: the so-called troll toll. If you feel a comment that's being downvoted deserves further discussion, you might not want to downvote it below that threshold even if you otherwise would.
I've also found it's a good policy not to downvote responses to your own posts; it helps keep conversations civil and it helps avoid messy and unproductive downvote duels in long threads.
I upvote responses to my own comments on principle. Unless they're nasty, but that hasn't happened yet on LW.
I've only recently joined the LessWrong community, and I've been having a blast reading through posts and making the occasional comment. So far, I've received a few karma points, and I’m pretty sure I’m more proud of them than of all the work I did in high school put together.
My question is simple, and aimed a little more towards the veterans of LessWrong:
What are the guidelines for upvoting and downvoting? What makes a comment good, and what makes one bad? Is there somewhere I can go to find this out (I've looked, but there doesn't seem to be a guide on LessWrong already up. On the other hand, I lose my glasses while wearing them, so…)
Additionally, why do I sometimes see discussion posts with many comments but few upvotes, and others with many upvotes but few comments? If a post is worth commenting on, isn't it worth upvoting? I feel as though my map is missing a few pages here.
Not only would having a clear discussion of this help me review the comments of others better, it would also help me understand what I’m being reinforced for on each of my comments, so I can alter my behaviors accordingly.
I want to help keep this a well-kept garden, but I’m struggling to figure out how to trim the hedges.