For some questions, I'm not sure whether I should down-vote it, vote to close or flag for moderator attention? Which one should I choose?
I've seen recently some moderator flags that weren't completely justified, so here is some general guidelines about when to use what.
From the general guidelines:
When should I flag?
If anything happens on our site that makes you feel uncomfortable -- that clearly does not belong here, in your opinion -- please flag it and bring it to our attention!
When should I vote down?
Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.
When should I close a question?
Questions that are sufficiently off-topic, as outlined in the FAQ, should be closed by casting close votes. Questions that are sufficiently similar to older questions should be closed by casting close votes.
Note that once you gain the reputation to vote to close, you should no longer flag for moderator attention to have questions closed or migrated. It is your responsibility to vote now.
In general, the point is that in the long-term the community should moderate itself. In other words, vote as much as you can (in particular, down-votes on questions are free, you won't lose any reputation!), when you have enough rep, vote to close.