Once your iOS developer program membership is setup, you can start updating details in iTunes Connect and start adding information for your app. Before you begin, you will need to have the following to hand:

• The date when you want to ship your app (you can set the latest date allowed and change it later)
• A brief description of your app that iTunes will display to customers
• An app icon (1024 x 1024 pixels) ready for upload
• At least one screenshot of your app ready for upload
• An internal version number for your submission
• A bundle ID that you've set to match your app ID

You can find out more from Apple's site here.

Before you start adding the above info for your app, you will need a bundle ID that will match your app ID. You may need the help of your developer at this stage. You can grant access to your developer by assigning them a membership role in your profile.

Once your bundle ID is setup and assigned to your app ID, you can start entering your app data as above.