Hi Dear,
1. Download "Form 2" from ICAI's website. Its within Members Tag- All Forms. Fill up and submit it along with the documents specified at the end of the Form.
2. It is not mandatory to hold membership number at the time of Campus. If you apply for membership before placements, then show its acknowledment receipt at the Campus. If you dont apply, then you will have to fill up a form of undertaking at that point of time.
3. At end of Form 2, two boxes are given for signature. One for old & another for your current. So change your current one.
4. Yes, download Form For Identity Card and fill it up completely leaving behing only Membership No. details.