Well, you should actually do a lot of critiques--more than what you get. You're going to take away an incredible amount from it because it takes the perspective away from the story.
This is a list of some things that you can look for:
Repetitions. Once you know what to look for, these are pretty easy to find in most fiction posted for critique. Someone says something, and then half a paragraph later, it gets said again. Or an unusual word gets used too many times. A big repetition word is "looked." I used it eight times on one page, including twice in one paragraph once.
Setting issues. Again easy to find once you know what to look for. Sometimes the story will have the character appear elsewhere without actually going there or walking through the furniture. It's very easy to ignore the setting completely.
Anything you're having trouble with. If you're having trouble with show vs. tell, critique for it to help yourself learn--even if you really don't know too much about it.
Also, resist the party line. There are a lot of times where someone does a prologue, and everyone says, "Get rid of the prologue" or "turn it into chapter one." Or writes in omniscient viewpoint, uses a dream sequence, or flashback. Instead, look at the writing, and if it doesn't work, try to explain why it's not working; if it does work for you, mention that, too. You learn something from that, too, and the writer gets good information.
And remember, you don't need to do a lot. I usually go through and pick out things like repetitions or anything that catches my eye, then give two or three overall things I noticed. Sometimes I provide links if there's something that might be helpful. I would strongly suggest that you do not line edit for grammar, spelling, and punctuation--it's a lot of work, and the writer will not learn from it. Instead, I'll just give them a comment like, "You'll probably get more critiques if you clean up the grammar."