best plugin for integrating authorize.net

I am developing a website for my client. It is a event management website. Each event will have its own price. When one will click on a event that will show some detail as pop up window, there will be one button to purchase ticket for it. After clicking on purchease if he is an existing user then he just need to login to make the payment. If he is not an existing user in that case he have to fill the registration forms and then make the payment.

What is going to be the best way to do this?

Thanks,
Adil