I am also getting my first payment this month and same kind of problems even I have regarding that secret pin number. But rest matters I can help you. First of all click on your name in the right side of your screen. There you will get a option as view profile in the center and click on it. Then Click on Gifts and Awards in the upper tabs under your status. There you will get an option of Cash payment I guess that will be the very first option. Then click on to details near that option. Scroll down and you can see the details of the payment and a place to upload your invoice after scanning it. Upload it just as you upload pictures to your resources by choose file option. Get to this link for more details How to receive payments?
Upload your invoice as soon as possible. Still if you have some problems then you can contact Dr Sanjeev (Sandy), the webmaster. He will surely help you out.
To set or modify an existing PIN you need to go to "manage account" i.e click on your name after you have logged in. The go to "Manage Payment & Revenue settings" and you can set your secret PIN there. It should be a four digit PIN. I hope you understand now. Free to ask if you still have any query.Regards, Vimal