I have a lot of sympathy. I still find it hard to judge reviews a lot of the time.
The system gives much less feedback/information than it would be useful to know, especially when you're just starting out.
My advice is:
1) skip what you're quite unsure of. Someone else will deal with it.
2) on the other hand, don't be afraid of making the occasional mistake -- we all do. Fortunately, things aren't only relying on you.
3) The two review queues you can access are ones that are often fine. Your task is mostly to notice the problem posts. One way is to look at it as having layers like an onion:
is there some very bad problem that needs to be addressed? (spam, something really offensive, a user under 13 giving personal information, or something else that clearly contravenes the user agreement)
For the big problems, most of the time, you just flag it, and move on. It's a moderator's problem now. If you can vote to close or whatever, feel free to do that. But the flag's the important one.
new users' first posts often need an edit. You can suggest edits. Don't worry about it if it's just one or two spelling mistakes in unimportant words. Make at least some substantive changes, rather than only cosmetic ones - though titles tend to be more important, so lean toward fixing those. Often posts need a lot of work -- suggest good tags, fix obvious formatting problems, try to make the post clear. If you do decide there's enough of a problem there to edit something, then fix everything you can (so once you're editing, then fix minor spelling errors too).
with first posts, you're also serving an educational function. New users often don't understand how things work. If it doesn't seem like anything you should flag or vote on (if you can vote to close, say), maybe you can post a comment to suggest something to the user. New users may not know where the help is, for example (even though they need it), or that you can't post questions in answers, or how community norms for homework go or any of a vast number of other issues. You have a chance to give the site a friendly face and the new user a nudge in the right direction.
don't pass up the opportunity to upvote a first question or first answer that's pretty good. If the poster has put effort in to make a decent question or answer, encourage them. If you suggest some fix to them (or someone else does) and they fix it ... vote it up then. Encouraging people when they're doing well is at least as important as the guidance we offer on posts with problems.
I don't think that you can vote to close yet, so you may not have that option (you'll see what you can do when you use the queue).
One thing you can do to see how posts were handled (whether you did anything or not) is to paste the URL somewhere (right click the title when you review it and you should have an option to either copy the address or open it in a new tab, or possibly both), and then come back a while later (or the next day if need be), and load that location to take a look at the post. Was it deleted? Put on hold by community vote? Held by a moderator? Edited? Commented on?
For example, while I was typing my answer, this question was in the first post review queue. Take a look at it now to see how it was dealt with. Two users voted to close as too broad (five users with enough privilege to do so can close a post that way), and a moderator agreed (so the post closed right away). There were other problems with the post of course, but the most important thing is dealt with.
You'll sometimes see things happen you don't agree with (it happens to me every day). If it's a really big deal you can either respond to a comment with polite comment in response (but the main aim with comments should be to do things to improve the post), or if there was an action you really think is mistaken to the extent that it's a problem, you could bring it up on meta.
