The Working with Visa APIs guide shows you the key steps for application creation, credentials, and connection validation.
After testing your App in sandbox, submit a Going Live request for evaluation.
Besides the steps above on how to get started, it looks like you're on the right track with asking your questions here in the Visa Developer Center community forum. You can also email your questions to firstname.lastname@example.org. Both of these channels have dedicated Visa Developer support staff that will be happy to help answer your questions.
Was your question answered? Don't forget to click on "Accept as Solution" to help other devs find the answer to the same question.