In Lightroom you can start cropping using the Crop Overlay icon in the Develop Module.
Alternative is using the shortcut 'R'. It sure speeds up the editing when you know your keyboard shortcuts. You can find the shortcuts in the help menu of by hitting CMD / or CTRL / if you are using Windows.
When you're in the crop overlay mode you'll see these lines around your picture. You can drag the lines in the way you want to crop your picture.
I know I talked about it before, but I'll say it again: in 99 out of 100 images you'll want to have a straight horizontal line. Lightroom has an easy tool to establish just that: the straighten tool.
Click on the icon and draw a straight line over the horizon. Your image is now perfectly straightened.
So now you know the technicalities on how to crop your picture. But what is a good ratio? Lightroom can also help you out here. Using the shortcut O gives you several crop ratios as options.
Each time you hit the O another ratio is displayed in your image.
For example rule of thirds:
By clicking SHIFT + O you can rotate the spiral 90 degrees
You can add or delete ratio's in the menu Tools > Crop Guide Overlay > Choose Aspect Ratios
This leaves you with a lot of possibilities to crop your image. And let me say this once again: there's no right or wrong here. Just try several things and see what works best for the image.
If I'm really honest, I crop by just looking at the image and feeling what fits best. For landscape images for example, I like it to have a panoramic look and feel. And for sports it's real easy: crop as tight as possible.
Last but certainly not least: don't forget to make a good composition when you are actually taking pictures. You can do your cropping with your camera, like when I shot the image of this triathlete, I worked with my 400mm and waited until the athlete filled the frame before I shot the image.
So go out there and have fun shooting great images!