1. Download a store certificate and provision file from Apple developer site.
2. Add code similar to this:
#If RELEASE
#CertificateFile: ios_distribution.cer
#ProvisionFile: store.mobileprovision
#END IF
This tells the compiler to use the store keys when you compile in release mode.
3. Tools - Build Server - Build Release App.
4. Tools - Build Server - Download Last Build.
5. Open <project>\Objects folder. You will see a file named archive.zip. Make a backup of this file.
6. Inside this zip file you find the ipa file. You need to submit the ipa file to itunes connect with Application Loader (on a Mac).
Before you can upload the ipa file you need to create a new app in iTunes Connect and fill all the required fields.