Best of luck to you in learning how to fly
I was in the same situation as you at first when I started lessons. However, I already knew a good amount of the phraseology and basically what to expect and what to say, but when you talk on the radio for the first few times, it is a bit intimidating. Inside, I felt a lot of mic fright, and my biggest problem was speaking back to the controllers what they needed to hear.
When I briefed with my CFI on the ground before we flew, as we'd go over the sectionals and routes, I'd ask about the communications aspect as well, "Ok, who am I talking to here? Ok, so I would say, '<insert pertinent ATC call here>'?" And he would tell me either yes, or he'd suggest an alternative. Then I'd repeat back the phrase that we both agreed on, if it needed changing. Then we'd move on to the next part of the route, and so on and so forth. It really helped me.
Listening to ATC on liveatc.net and listening to it in the cockpit are two different things for me. Like others have said, one tends to be so involved with the plane, that they don't have enough brain cells to devote to communicating. But soon, your muscle memory will be trained to fly the aircraft, you won't be hunting for an airspeed or an altitude, and things will just...flow. It's at that moment that you'll really realize what an awesome thing it is that you're doing. ATC fear will be a thing of the past.
My advice in addition to the others:
1.) If you can find a feed that records where you fly, try to listen to yourself and see how you sound.
2.) Record yourself talking and then play it back. I sometimes practice my radio calls while on my commute to work, lol.
3.) There's a lot of repetition in what ATC says. They stick to many key phrases, and only deviate slightly. Commit the major phrases to your memory, and you'll be able to pick it out even from the fastest talking controller.
4.) Sound CONFIDENT. If you sound confident, the world is at your command.
5.) If you tend to go "uhhh" and pause a lot, break that habit immediately. Prepare to say what you need to a few seconds before you click the mike switch.
6.) Stay ahead of the aircraft. That means, plan, plan, plan. Know where you're going before you get there. Things tend to go haywire when you get sidetracked and have to rush to make up for the difference.
7.) You really don't have to say you're a student on initial callup, unless your really struggling or you feel you will need the help in the relatively near future. At least that's my opinion. But, be true to yourself. If you feel more at ease by telling them you are a student, by all means, do so. There is no shame in erring on the side of caution.
8.) Ditch unnecessary words and phrases. Convey what you need to while being concise and polite.
9.) Have the desire to be bulletproof in your coms. Clearly you want to be good at this, so keep practicing, just like you do your flying, and you'll be a master in no time. It will get easier! Good luck.
Here's a good article on communicating, hope this helps you as it did me:http://bathursted.ccnb.nb.ca/vatcan/fir/moncton/WeeklyTopics/Archives/20040502/CurrentTopic.html