It took me a while to dive into the C&C part when I started. Believe it or not, I was a little shy back then.

I started by giving C&C on things that I was working on. Let's take the ROT's for example. I would always look through the posts and see if they used it, or if the shot would of been better off if they didn't. I would comment about it when appropriate. Eventually, I had a long list of things that I looked for in a shot and was able to comment about them.
That worked for me and doesn't have to work for anyone else. =)
I think everyone will be different. Never feel you shouldn't jump in and contribute!
^+1 to this, except the part about mish ever being shy...
I still wouldn't say I always FEEL qualified; it just depends on the photo. If it's street photography, I'd be extremely hesitant to give any real critique, because I know little about what makes good street photography. If it's a bird in flight, though, I'd feel much more confident lending my thoughts.
To my way of thinking, giving C&C is much like learning photography, in that you have to start DOING it at some point if you really want to LEARN it. You can read about photography, ask questions, look at photos, etc--but if you want to become proficient, you have to practice actuating that shutter. The same is true of giving critique--you have to practice actually doing it.
When I first joined TPF, and for probably at least the first year, I almost never gave any C&C for the same reasons you've mentioned--basically, I wasn't confident that I really could give any decent critique, my C&C would get ripped apart, and then I'd have to cry myself to sleep. :lmao:
What I did to get over my fear of giving C&C? I would look at ONLY the original post and then I'd "write" a C&C response, in my head, trying to be as specific as possible. THEN, I'd just keep an eye on the thread, and note the responses given by particular people whose opinions I really respected. Discovering how often those people would make comments very similar to what I'd thought helped give me the confidence to start actually adding my C&C to a thread.
I also did this with some threads where all I could think was that I liked the photo or didn't like it, but couldn't express why. Reading the critique from others often helped me pinpoint the reasons I felt the way I did about a photo, and eventually I got better at *seeing* those things for myself. It's a learning process, and even if you're off the mark once in a while, it's not gonna kill anybody!