Subscriber Identity Modules (SIM Cards) are used in Orange's Global System for Mobile Communications (GSM) cellular mobile phone handsets. They contain the phone number. Registering an Orange SIM card connects it to the Orange network. Once you've registered the SIM card you become a user on the "Orange Starter" plan. You can then choose from one of the pay-as-you go plans that may be targeted better to your style of use---like cheap international calls or unlimited free texts.
Insert the SIM card into your phone and turn the phone on.
Browse to the Orange Welcome to Pay As You Go website page (see below.) and click on the "Click here to get started" button at the bottom of the page.
Enter your Orange phone number on the Register Your Pay As You Go Phone or SIM page. Use the phone number's leading zero, for example "07" and then the rest of the number. Press the "Next" button.
Follow the rest of the steps on the website, entering the requested information at the prompts. Press the "Next" button to proceed through a total of four pages.
Monitor your phone for a text message and follow any instructions in the text.
Alternatively, if you don't have your Orange phone number, you can use the SIM card number. The SIM card number is printed on the SIM card packaging, on the outside of the box the phone came in, and on the SIM card itself. Simply select "I don't have my Orange Mobile Number" on the Register Your Pay As You Go Phone or SIM page. That's the first of the four pages.