Student or Learner
Is the following sentence acceptable?
I wish my advice in this letter could help you to be more prepared before you come to America.
To say it this way "I wish my advice ... could help you..." implies resignation, that, alas, you wish it could, but it won't. I'm sorry. Maybe someone else can give you the advice you need.
You may be trying to say something more hopeful, more optimistic and welcoming. In this case, I would suggest "I hope my advice here helps you prepare for your arrival in America."