Strictly speaking, I don't think "niwa" and "dewa" are particles. They are "ni" and "de" followed by "wa", which is the topic-marking particle. So the difference between "niwa" and "dewa" is the same as the difference between "ni" and "de".
I think the main difference between
ni and
de when they're used with a location is that
ni marks where something
is and
de marks where something
happens. Unlikely example follows:-
にわ
には犬がいます。
In the garden there is a dog.
にわ
では犬が猫を食べています。
In the garden there's a dog eating a cat.
Until your reply ありがとうございます。
On to the nit-picking! You can't use ありがとうございます to mean "thanks in advance". The literal meaning of ありがとう is something very roughly like "you did a great job". It just doesn't work until after the great job has been done.
On the plus side, this is a good place to try out よろしく. It's a difficult word to pin down (i.e. I don't really understand it) but some of its meanings include "I'm relying on you", "I trust in your support" and even "thanks in advance".
マイケル