Can you please let me know the specific payment API that you are interested in using for your business project? The level of difficulty varies with what you are trying to do, the API that you are using for your project, knowledge of APIs and integration setup, etc.
In particular, if you are interested in using Visa Direct, see the top few paragraphs that are shaded in yellow that discuss the requirements to use Visa Direct.
After having read the documentation, if you meet those requirements, then please click through the link in those paragraphs to the Visa Direct Program Implementation Questionnaire, fill it out, and submit to the email address listed on that form.
To get started, please follow the steps in the “Working with Visa APIs” link below to register and create a Visa Developer application.