Go forward twice. Then on the right side hover over the line of people. If you hover over the right witch you should see a bit of paper. When you click on the bit of paper, it will be your shopping list, you will unlock Gringotts.
When you get to the Diagon Alley chapter, double click, twice, on the crowd to make them move away. There is a tall white building ahead - that's Gringotts. You have to click the door.
If you have already unlocked it, you need to wait until you absolutely positively need to buy something. The key will appear when buying something is necessary on the chapter. You can't just buy anything. So, drag the key into the keyhole (I think you worked that out when you first got it). Good Luck!
You just click the building in the back of the north alley (or south whichever is there when you first enter Diagon Alley) all it does is give info and tell you how much money you have.
You can find a guide on the Pottermore Wikia.
Yes, you can find many things. For a guide per chapter see the Pottermore Wiki.
The key is silver with blue wings, one of which is broken.
In Bellatrix Lestrange's vault in Gringotts
Double click and it will be on the floor.
Harry gets his wizarding world money (galleons, sickles, knuts) at Gringotts, in Diagon Alley.
The aim is that you must find your way to pottermore. On that site you will find the magic quill, clicking that will let you register.
Gringotts (the wizarding bank) was not from a film studio. Gringotts Bank is actually Australia House in London.
Gringotts Bank is run by goblins.