I brought my house from the Australian shop Adairs Kids, it started out looking pretty cute in pink and green, but I'm not really a pink kinda girl ....
If you want to put up wall paper and change the flooring now is also a good time to look at doing this. I found it easier to do walls and floors while it is flat, although this house shape is very simple and walls and floors and easily be altered once the house has been put back together.
I generally just tack wall paper to the house using blutak as a I change my mind.... a lot! But any craft glue will be sufficient to get the paper to stick to the walls. Cardstock or scrapbooking paper is the best type of paper to use on the walls as the glue will not transfer through and ruin the pattern on the outside. For one of the main rooms I used a white brick patterned scrapbook paper from Australian store spotlight and the flooring is a roll of wood patterned adhesive I picked up from Bunnings.