Presumably these are going to companies? Or is this for private individuals only?
Start with a brief account of the charity and what it does.
Go on to explain the reason for needing to raise funds.
Then explain how donations of bric-a-brac/clothes/household items can be used to raise these funds.
If this is for private individuals, tell them where and how the donations can be made, or explain the means by which the donations can be collected.
End by saying "Thank you for any contributions you may be able to make".
When you have written your letter, post it and see what we have to say