I've already set all de iTunes Connect stuff(Snapshots, Description, Tags, etc...) and downloaded the Application Loader but it asks me for an .app file and I don't know how to get the .app file from my Xcode project or if I need a Certificate to do it or something please can someone help me?
A great place to look for answers to questions such as this is the Apple Documentation
In short, you need to:
The documentation linked above will guide you through this step by step.