Your setup might not be as complex a project to require a mixer or an audio interface. An audio interface simply serves as the hub to get audio from multiple pieces of equipment (such as a guitar and an external mic) to your computer. If you don't need to hook multiple types of equipment to your computer (like if you're a solo singer), you might not need an audio interface.
We encourage you to keep that in mind before purchasing an audio interface. You can always check your computer to see which types of ports your computer supports and confirm if investing in that equipment will be worth it.