I'm Miles from the Social Care team at Shopify. Thanks for reaching out. It's great to hear you've found a payment provider you'd like to utilise. When you choose your payment provider, it's likely they will require you to create an account with them for you to be able to connect your Shopify account with them. Have you already spoken to them to obtain the credentials? Depending on the provider, they may require a username and password, API key, authorisation key or all of the above.
Shopify Payments is the best solution to quickly getting online. It's built-in to your admin and you can apply right from your Settings > Payments screen if you're eligible. You can check out the requirements for Shopify Payments here. Once you fill out the application, you'll be able to start accepting payment straight away.
Please let me know if you run into any further issues.