The .p12 file saves your certificate and private key information. Creating the .p12 certificate requires you to use the Apple Developer site to create and download a certificate and then use the Keychain Access utility to export the certificate and private key of that certificate as a .p12 certificate. Follow these steps to set up your provisioning profiles and creating your .p12 certificate.

Go to the iOS Provisioning Portal, log in using your credentials, once in click Provisioning in the left menu and then select the Distribution tab.

You are now going to create two profiles, one for reviewing your app and publications (Ad Hoc) and one for App Store (App Store). Click New Profile.

First the distribution profile for AppStore.

- Distribution Method - Check App Store

- Profile name - Enter the name of your app, add AppStore in the end.
E.g. AppName AppStore

- App ID - Select the App ID from the dropdown list of the app you are creating the provisioning profile for. Click Submit

Now we are going to create the distribution provisioning profile for AdHoc. Verify that you have all the test devices listed under Devices. If you don’t, add devices by following the steps under “Test Devices” on page 5. 

- Distribution Method - Check Ad Hoc

- Profile name - Enter the name of your app, add AdHoc in the end.
E.g. AppName AdHoc

- App ID - Select the App ID from the dropdown list of the app you are creating the provisioning profile for. Click Submit

- Devices - Select up to 100 devices for distributing the final application; the final test application will run only on these selected devices.

- Click Submit. 

Now click Certificates in the left side menu and select the Distribution tab. Now you should see your two new profiles under Provisioning Profiles.

Before you continue you have to have the WWDR intermediate certificate installed, you can download it using the link under your certificate.

When you have installed the WWDR intermediate certificate, download your Distribution Certificate and follow these steps:

- Open your Keychain Access application (found under Applications / Utilities).

- Select the Keys category.

- Control-Click on the private key associated with your iOS Development Certificate and click Export Items in the menu. The private key is identified by the iOS Developer: <First Name> <Last Name> public certificate that it is paired with.

- Save your key in the Personal Information Exchange (.p12) file format.

- You will be asked to create a password but leave it blank and click OK.

- Now you have created your .p12 file to upload to Publish.