Hi Kyouko,
Also, you often want to use "such" with an additional explanation, remember "such...that..."
For example, all of these are ok to say by themselves:
This piece of cake is big.
This piece of cake is very big.
This piece of cake is so big!
This piece of cake is so very big!
When you use "such", you can explain or compare with "such...that..."
It is such a big piece of cake that I can only finish half of it.
It was such a big piece of cake that I gave half of it to my sister.
This is similar to Micheal's explanation for "so" above.