Identifying mushrooms by photo is difficult. You really need to take a spore print and use a keyed guide. A book I like is the Lone Pine Press "Mushrooms of Ontario".

Below is a link to a key to gilled mushrooms, gilled mushrooms are the largest group and have fan like gills on their undersurface (common grocery store white mushrooms have gills).

If your mushrooms do not have gills, go to the home page of the site and read some of the other links to narrow down what your mushrooms might be and try the other keys.

If you have a photo, you could try posting on the Fungi forum, be forewarned that there are many,many, many white mushrooms.

Here is a link that might be useful: Gilled Mushroom Key