Cropping is straightforward for the most part, but it can be tricky at first, and especially so if you're looking to crop for printing purposes. This video gives you a great, beginner's guide to cropping your photographs for all purposes using Adobe Photoshop.
Cropping your photographs in post-production is one of the fundamental tools every photographer needs, but it isn't always straightforward. If you're looking at turning your 16:9 ratio landscape into a 1:1 square crop, there's little to know, but when it becomes more intricate than that Photoshop has a number of tools to help. These tools are particularly useful if you are looking to print, where the aspect ratio your image is in is only a part of the whole process.
It may seem absurd, but cropping images is one of the most satisfying components of post-production for me. I often shoot on a medium format body, and so I have a lot of wiggle room with resolution. (That said, most modern cameras, regardless of sensor size, can crop in comfortably.) On a recent trip, however, I found that I was cropping a lot of my drone shots for better composition, and working my way through a lot of images meant full control over shortcuts and tools sped up my workflow significantly.