If you want to integrate with Marketo via the ReST API, best practice is to create an API only user role, Create an API Only User and then a Create a Custom Service for Use with ReST API. Here is how to create the API only user role:

Under Admin, click on Users & Roles.

Click on the Roles tab and select New Role

Enter a Role Name, select the API access Permissions you want to give and click Create.

OK, well done! Now let's Create an API Only User.