If you want to crop an image, just upload it through the entry form or in the assets area. Then go to the assets area (1), browse to the correct folder (the image below is just an example, it can be any folder) and select the image you want to crop (2). Then click the gear icon on top and select Crop image (3).
A pop-up will open in which you can select your crop size (aspect ratio). If you select free, you can drag the handles on the image to select the crop you want.
Please take care (especially for thumbnails) that the dimensions are already kind of in the right direction. For thumbnails on the website, images that have a 16:9 aspect ratio work best!