Where are you having your party? Does it have a plain white wall, a brick wall, or a super colourful wall like the one above? If you have a venue with any kind of blank wall, make this your designated photo booth and you're already one step ahead.
If it doesn't, then you will need to buy a backdrop. Our tip for getting one cheaply is to buy either a large piece of fabric or some poster paper. If you have a theme in mind or want something really jazzy or patterned, go with the fabric. You can pick up really large sizes on Ebay for under £5, and for a few pounds more, you can get something fun like a holographic fabric. We love sparkly fringe curtains, too: they're cheap and look amazing in pictures.
If you want something more simple, then go for extrawide poster paper; it looks great and is sure to give you that photo booth feel. Whatever you pick for your backdrop, you're going to want it to be extra wide so that it fills most camera screens. Don't worry if you order too large, as it's easy to trim the sides of both fabric or paper.