If you want to integrate with Marketo via the REST API, you'll need to create an API only user. Here's how.
Under Admin, click Users & Roles.
Click Invite New User.
Enter an Email, First Name, and Last Name for the API only user. Click Next.
Select the API Only role and check the API Only checkbox. Click Next.
The pop-up says, "An invitation is not required for API only," but that doesn't mean you've done something wrong. It just means we'll create the role without an invite email having to be sent.