How can I effectively test a new payment app on Shopify?

[ahsan-kinectro]

Hi I am new to shopify as a developer. Basically we want to create a payment provider in shopify. I have read the documentation and found out we have to make an extension for a payment app. We create connect and publish app.But we did not found this working and some links on testing payment app section are not working.So how can we see that what we are doing wrong and one more thing they ask to give credential detail in app listing but the asked for payment first to start the plan. Actually my question is that Is it must to buy a plan even for testing in development environment or we need some third party integration and also provide me with some helping material if there is any. Thanks