1. Class-3 DSC (Digital Signature Certificate) 
2. Mobile Number and Email Address (email address must be genuine, it can't change in future and all information will be send to the email address) 
3. GST certificate 
4. PAN card of firm
5. Aadhaar Card of applicant
6. Passport Size photo (1 photo, but it's better to keep at least 3 photos) 
7. Stamp of firm
8. Declaration form that is available at www.ireps.gov.in (this can be download even at time of registration) 

Note: you will get login User ID and Password in your email address that you provided at the time of registration. You will get login username and password in around 3-4 days. I got my registration many time in 3-4 days. But you should do everything in advance (because it's Indian Railways