Excellent advice so far, but I can add still more!
Firstly, your instructor should always be the first port of call if you have any questions. If he's aware you're unsure, or concerned about something, he can concentrate on it over the next few lessons, and do some extra ground school with you. Don't be afraid of looking silly!
From your other posts I'm guessing you're pretty early on in your training? Don't start worrying about whether something
might be a problem in 15 flying hour's time! There are only a handful of calls you'll need to make before you get onto navigation. Why don't you get together with another student from your school/club and practise them on the ground. Draw a circuit and take it in turns being ATC and pilot. You'll find the calls much easier to make when you know what you expect to hear when.
When you do get onto navigation you can do the same again. Draw out an imaginary route, and practise making the calls, taking it in turns being ATC and pilot. You can practise position reports while you're out and about during the day, walking down the street, sitting on a bus etc. *hint* don't speak too loud
Also, you can write yourself a reminder about what information needs to be passed, and slip it into your kneeboard. Try not to use it, but if you get stuck, it's there as a back up.
Why don't you get yourself up the tower and have a chat to the ATCOs? Not only will you realise that they're almost human, but next time you're having a flap on the radio you will be able to put a face with the voice, and that should relax you a little.
Also remember that ATC are there to serve you
They would much rather hear you say "Umm, I'm really not sure what you want to know, or where I am" than a guess at both! The sooner they know you've got a problem the easier it will be for everyone to fix it. Yes, you should try to stick to protocol, but no-one's going to shoot you for saying "G-** could you just remind me what you want me to do next?" It's better than not doing what they want you to do because you're afraid to say you've forgotten!