It's a pretty common piece of advice to new writers: "Get a critique group." But how useful, really, is a critique group? If you follow a few rules, it can be incredibly helpful - if not, it can be one of the most painful/boring/torturous experiences of your life.
First of all, don't go in blind. In my experience, critique groups work best when everyone is writing in the same genre (i.e. "fantasy") but the pieces are not so similar that people worry about being copied (i.e. "we're all writing coming-of-age stories about teenage girls set at a fantasy boarding school!). That's not to say that you can't critique someone's futuristic space opera if you write historical fiction; if that's what you're doing, make sure all participants have read in that genre before so they know the conventions and can understand how cleverly you've subverted the tropes.
Second, it's okay if you're acquainted with the people in your critique group outside the group, but if you want to have any kind of productive experience, don't start a critique group with your three best friends. It's really tough to give an honest critique of a piece - that is, give it the help it needs - if you know how your friend poured his/her heart into it and will be crushed by criticism. My first real critique group, in my freshman year of high school, was a group of close friends, and we had some real problems with hurt feelings.
Third - related to that - be honest, but be kind. Don't lie or leave things out (that's just unproductive), but don't give someone a critique letter that says "this sucked". Balance your criticism with tact, and if you don't like something, try to articulate why it didn't work.
Workshops - by which I mean the kind of college writing workshop you'll probably experience if you take any creative writing classes ever - are a slightly different story. You have no say in the members, and there's no way to know what they read. Take comments with a heaping cup of salt. Sometimes you do get very useful feedback - and sometimes you get a bunch of students ripping your piece apart to prove how smart they are. Even if people "don't understand" something, their opinions can give you helpful perspective. Occasionally.
I have only one hard-and-fast rules for critique groups and workshops: for the love of God, proofread your piece before you turn it in. Yes, sentence fragments can provide a nice effect, but your story needs to be readable. Nothing is worse than trying to critique a story and noticing that only half of the sentences are even coherent.
As for beta readers - individuals who read your work before anyone else - I'm a big fan. Pick them well, treat them well, and they will save your life (and sanity).