why is it everytime I'm downloading a new item/games it always replaces something?

You can only store two games on your Tamagotchi iD at any one time (not including GoGoSlot and Catch).

Maximum Download Storage

Location___________Maximum number of downloaded items you can storage there
Restaurant__________6 meals
TamaDoughnuts_______1 doughnut flavour
TamaCafe____________3 snacks
Game Center_________2 mini-games
Gotchi Interior_____4 livingroom wallpapers
TamaDepa____________4 toys
Photo Studio________5 costumes and 1 backdrop
Accessories Shop____3 accessories
Door Menu___________2 destinations
Mailbox____________10 letters

can I know those destinations?
and where can I download them???

So far only one destination has been made available for download.

The download link for it is in this post: http://tama-zone.com/index.php?showtopic=2...dpost&p=3383170

how exactly can I make my own wallpaper?
and how do I make my house bigger? (outside) beside the mailbox?

For custom wallpaper, check this topic: http://tama-zone.com/index.php?showtopic=2...dpost&p=3368247

The only way to change the outside of your house is redecorate every room inside your house (living room, bedroom, kitchen, etc.) as sweets style. Note: if you then change the living room to a custom wallpaper, the outside of the house will revert to normal.