Hi narniagirl87! ;)
Firstly, welcome to Fanmai.biz! {clap} I hope you enjoy it here!
Write the letter from your heart, so if you could tell Will something, what would you say? Try to keep the letter short to one page only - only say the necessary things.

You can write about why you admire him, what your favourite film is and why, etc. You can also tell him about yourself, anything you think he would want to know.
Will does have his own photos to send to fans, so it's up to you if you want to send a photo or not. {thumb2}
Lastly, you may send a small ID photo of yourself and paste it onto the letter. There's nothing wrong with that, I'm sure celebrities wonder what the person, who wrote the letter, look like.
Feel free to ask if you have any more questions!
Good luck,
zanne