JQuery: Validate Certain Fields With A Function In A Multipart Form

I have a simplified version below of a multipart form. There are two steps and two tabs. I want the user to enter their username and email address and click 'Next' which will validate to make sure those fields are filled in, then activate and advance to tab #2 (Step #2) of the form, where they enter their credit card information.

I am having difficulty because username, email and credit card are all in the same form and I need to only validate certain fields on each step of the form. I was thinking I could make a function when the 'Next' button was hit that would validate the first two fields?

Here is what I have so far. I have a 'Next' button which activates and advances the tab to Step #2, but I need to work some form of validation into it before it advances. At the bottom of the page I have a 'Submit' button which is just so you can see that the validation is working if the form is submitted. I just want the 'Next' button to activate a function to only validate the fields on Step #1 (username/email).

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