Well, if you own the house and can modify the walls, all you need to do is to find a pattern you like. If it is not in your inventory, put it in there and in your texture folder. If you find something online you like, upload it by pressing Control + U. It will cost you 10L.
Then right click on the wall(s) and select texture tab. Click on texture and chose the pattern from your inventory. Voila.
If you can't modify the walls, the easiest thing might be is to make a flat sheet to cover the wall. Then add the texture to the sheet. This will work on a wall with no windows or doors. Otherwise it gets more complicated. Making the flat sheet will mean using the Build function in your viewer, so you need to be somewhere where that is allowed.
If you need help inworld, let me know here or IM me or sent me a notecard.