Is it an iPhone 4 or newer? If so, Apple has made it hard to repair them yourself by changing the screws to a torx head, I believe.
Same thing happened to my iPhone 3GS. I ordered an $8 screen repair kit from Amazon, which came with everything I needed. It took me 3 hours to change it though, and was the hardest thing I've ever done. It required me to almost completely disassemble the iPhone and put it back together. One of the tiny screws stripped out so I had to drill it out with a tiny drill bit.
Your best bet would be to schedule an appointment with the Genius Bar at Apple Store. I think they can fix them very quick. If they can't they might just give you a new phone for free - because their main job is to get you in and out in less than 15 minutes.