A mockup is an accurate static design of a final product.
A mockup shouldn’t just outline the layout but also present the content and functionality.
Unlike wireframes, which just give an idea of the layout, a mockup should look like the final product. A mockup is, like the wireframe, just a static presentation, this means it is not interactive and clickable like a prototype.
You can use a pen and paper and draw your ideas.
But to create some more realistic mockups, you need mockup tools.
There are plenty of tools free and paid for. Some free tools you could try are AdobeXD and UXPin.
Paid tools are, for example, InVision or Moqups. If you need some example mockups, or want to get inspiration, you can take a look at dribbble.