It was wonderful news when I received the letter that informed me of "your acceptance of me in your teem as a trainee" from July 1 to ...
I was delighted to receive your offer for a traineeship in your company from the 1st of July to...
Ce n'est qu'une suggestion, je pense que tu peux trouver mieux, mais souvent en anglais plus simple est le plus efficace (enfin je trouve

)
Autres suggestions (pas forcement parfaites!)
Do let me know = let me know
If I can do anything in advance of my start date = if there is any preparatory work that i could do prior to my starting day
or if there are any areas = any topics
Pourrais je te demander par curiosite comment tu as obtenu ce stage et aupres de quelle compagnie? J'ai une amie qui cherche un stage en compagnie de traduction en angleterre et qui galere
Bon courage pour la suite!