As with many cases, smartthings is very powerful, but not always very intuitive.
You can use the official smart lighting feature to set up a virtual timer for a light, but you also need to create a virtual switch to act as that timer. There is a how to article in the community – created wiki with the instructions:
You could also definitely use core if you want. Core is a very sophisticated rules engine created by the community. But it does require that you install custom code and it may be overkill if you just need a timer for a couple of lights.