I'd like to begin by welcoming you to the QuickBooks Family. I'd be more than happy to help you with adding a payment account so that you can have customers pay with credit cards.
With just a few easy clicks, you can sign up for a payment account from within QuickBooks Online. When you open up an invoice, you should see an option at the top of the screen next to Online payments that says Get set up in blue. Selecting this link will open up a prompt and selecting Finish setup will open the page for applying for an account. I've included some screenshots to serve as a visual guide:
Should you have any questions about the application process, I'm including the contact information for the QuickBooks Payments Phone Support Team: Contact QuickBooks Payments. This team can best assist with reviewing your account application, as the sharing of sensitive information may be required, and this is a public forum.
You referenced that you just installed and are setting up your account and I want to ensure that you get the most out of QuickBooks for your business. The following link contains tutorials designed to help a user get acclimated to the software: QuickBooks Tutorials.
I'm just a post away if you need further assistance. Cheering you to a pleasant weekend ahead.