When you first get to the Resort Area in Sinnoh, you will be shown to a building by a person there. They will say that this house is now essentially yours to do with as you please. Inside, you'll just find a single table. However, there's much more going on there than what meets the eye

When inside, you will see a table, and on this table is a book that allows you to mail order loads of new items to kit the house out with. Unfortunately, these items are very very expensive. These items are as follows;

Pic Name Price Available From
Table
Johanna will visit		 Free From Beginning
Big Sofa 120,000 PokéDollars From Beginning
Small Sofa
Dawn/Lucas will visit		 90,000 PokéDollars From Beginning
Bed 187,000 PokéDollars From Beginning
Night Table 140,000 PokéDollars From Beginning
TV 220,000 PokéDollars From Beginning
AudioSet 160,000 PokéDollars From Beginning
Shelves
Prof. Rowan will visit		 150,000 PokéDollars From Beginning
Racks 127,000 PokéDollars From Beginning
House Plant
Gardenia will visit		 120,000 PokéDollars From Beginning
PC Desk 168,000 PokéDollars From Beginning
Music Box 25,300 PokéDollars Purchase Racks
Pokémon Statue 150,000 PokéDollars Obtain a Silver Print in the Battle Frontier
Pokémon Statue 150,000 PokéDollars Partake in all 5 facilities of the Battle Frontier
Piano
Cynthia will visit		 146,700 PokéDollars Defeat the Elite Four 10 times
  Reception Set 208,000 PokéDollars Defeat a total of 50 trainers in the Trainer Café in Battleground
  Wall Clock 52,000 PokéDollars Plant 50 Berries
  Great Painting 140,000 PokéDollars Hatch 30 Eggs
  Tea Set 108,000 PokéDollars Purchase Reception Set
  Chandelier 108,000 PokéDollars Walk a total of 300,000 steps

Thats not all though, as your house will be visited from time to time by a number of prominent main characters you encounter throughout the game including the gym leaders, your rivals, Cynthia, Professor Rowan and your mother. They will appear randomly after you have bought the item that activates them and they will then appear from time to time.