The one thing you have to consider about repairing your iPhone is that you'll void the warranty and Apple will not service it in the future. If you want to preserve your warranty, you'll have to have the repair done by Apple or an Apple authorized repair centre.
A screen bought from eBay may be cheap but there's too much chance of getting a counterfeit or defective part. You should purchase parts only from reputable sellers that sell only guaranteed original Apple parts.
If you've never repaired an iPhone before, you should seriously consider having a reputable repair shop do the repair.
ifixit.com sells genuine parts and the necessary tools necessary for iPhone repair. They also have repair tutorials on their site.
