Hi Eysha, welcome to the forum
I've written a tutorial which explains the basics of exposure which you can read
here.
With regards to the lens you've bought, I assume it screws onto the front of your current camera and has a 55mm thread diameter. If thats the case, it will fit to all sorts of SLRs as it screws onto the lens and not the camera directly.
As Steve mentioned, there are also adaptors which allow you to fit it to lenses of different sizes, although using it on a lens of a larger size (say a 67mm lens) would give you some rather odd results.