I suggest thinking on what you want to build. You can't just go wacky-doo on your interior and it does not match the exterior. For example, you can't go build a medieval interior in a modern skyscraper. Well, you could but that would look weird, imo.
Once you have decided your theme or the style for your build, take a look at some pictures of the interiors of a building that your theme fits in. If you're building modern, take a look at some modern houses; medieval/vikingvhouses for medieval and so on.