T-shirt mock-up templates are a great resource as they tell you how your designs will look when freshly printed on garments. It can include a model wearing the product to help customers envision what the t-shirt looks like on, or may just feature the t-shirt on a plain background like in this one. Each template is very easy to use, and you can easily apply your designs to these mock-ups. You can edit the mock-up to whatever suits you by placing your designs via smart object.
A special thanks to Graphic Burger for the mock-up. A whole lot more are available on their website, so check out their website for more designs.