It's very easy for non-Europeans. You must call this number: 0845 641 5008. They are helpful and speak slowly. Give them your entire name and address. In a few days you will receive at home a form. It's very easy to complete it following the instructions attached. Send it using the envelop that you received with copies of your visa and passport informations. In a few days (one week or fewer) you will receive a letter with your Insurance Number!