procedure for payment gateway?

what are the documents needed to start payment gateway, how many days it will take to obtain merchant account.

Documents vary on the payment gateway requirements. You should visit the payment gateway website & read their requirements. As far as my knowledge, CCavenue needs your business name, bank account with your business name. you PAN number if based in india. or may be other needful docs you'll find all things clearly mentioned in the payment gateway site which you are going to use.

Well, I have opened a lot of merchant accounts for my projects, from my experience basic requirements are:
- Incorporation documents
- ID card/Passport
- Utility bill
- Website (you will need to indicate contact info on the websites (company name, address, email and phone))
----------------
Or you can go with Paypal, It is easier to start with. You can even start as physical person, but after some time you will need to switch to corporate account and prove everything.
----------------
It is quite complicated question, depends on product and payment methods you want to receive.

Yeah as already mentioned here, you will need incorporation documents and all personal docs i.e. passport copy, utility bill, bank statement and sometimes even reference letter. You need to prepare a business description and even draw the steps when a customer will buy from your website. There is a whole lot depending on what payment processor you are going to approach and if it is a direct merchant account or some 3rd party billing.