The hardest part for me is just read backs. If I read it back instantly while its fresh in my brain, I forget what I just read back. Damn, I just read it back, what was that squawk code again!! If I jot it down real quick, I feel like I'm leaving them waiting too long.
I'm not sure what kind of transponder you have, but I always punch the numbers in my Garmin as ATC is giving me the squawk code then reply "Cherokee 6966W squawking 1234". It is in the radio that fast and I just read back the numbers showing in the display. It makes for a good verification that I entered it correctly, instead of repeat back followed be entering the code afterwards. You can repeat the number back correctly and still potentially enter it wrong if you do the repeat back first.
It is natural to be nervous at first talking on the radio. When you start learning to fly you usually feel like you are operating in overload mode and talking on the radio is one too many tasks to have on top of controlling the plane. In time you will build your skill in both handling the airplane and the radio.
One of the things that helped me get over being nervous on the radio was realizing that the voice on the other end was just a regular person that was there to provide assistance. Nothing really to get nervous about. They would not have a job were it not for pilots like us.
Our EAA chapter invited some of the controllers from our local class "D" airport to come make a presentation at the meeting one Saturday. It was great meeting them in person and getting to know each other a little bit. It is especially neat when you recognize that familar voice on the other end when you establish communications with them. From 2012 thru 2014 I was working a consulting job about 125 miles from home. The local ATC got use to my schedule and would be expecing me on Tuesday mornings and Friday afternoons. After I finished that contract and started working close to home, I didn't fly for a few weeks because my airplane was in annual and I was making some business trips on commercial flights. When I was flying home after the annual inspection, as soon as KCAE passed me off to KAGS I called Approach. ATC response was "Where in the world have you been? I was use to handling you every Friday afternoon and was wondering what happened to you." So I'm flying along and we are having a brief conversation about my job change.
Needless to say ATC was not very busy that evening allowing us the opportunity to talk beyond the normal formal communications.
Bottom line is, yes you will be nervous at first and it will be a pain having to communicate while trying to fly the plane. With practice you will get comfortable with both over time. As others have already posted, just let them know you are a student and they will usually go out of their way to help you out and they will be more patient with you.
Good luck with your flying. And most of all, have fun!