Ideally, the bottles should be kept somewhere with a constant cool temperature of no more than about 10C but, in practice, any fairly cool room (such as an unused spare bedroom) is fine and, in most cases, you can keep them for up to a few months (at least) almost anywhere in your house.
The bottles should be stored away from light but that won't be a problem if they're boxed anyway. If you were to store them for several years, then they should be kept (like any other wines with a cork) on their sides, with the neck slightly lower than the base, but for a shorter period you can simply keep them upright. (It's actually quite hard to do any real harm to a bottle of bubbly!)
Unless you know that, once opened, you'll be consuming a whole bottle at once, ensure that you've got a proper champagne saver ready to cap the open bottle with:
https://www.waitrose.com/ecom/products/waitrose-champagne-saver/763692-625262-625263