Adam,
Don't worry, we all have our moments on the radio. When first starting out, as soon as you press that 'mike' button, your mind will almost guaranteed go blank!
Just practice the radio with your instructor on the ground or even just read about it at home and get to grips with everything.
Plan exactly what you are going to say before keying the mike. Just remember that the controllers are only human and they won't eat you alive if you make the odd mistake (they will know if you're a training flight!). Your instructor will also point out any errors and its just a case of taking it all on board and learning from your mistakes.
A bit more practice and you'll be on the radio asking for clearances, frequency changes etc without even giving it a thought.
Champair79