Show Me is a special menu on the top-right of the toolbar. You can easily spot it with the 
icon. If you click on the icon, you can open and close a list of all the predefined Show Me visualizations.
In the Show Me menu, you can see 24 visualizations. If no fields are selected in your Data Source, all the options are gray. If you choose one or multiple fields (using Ctrl or Command), you'll see that some options are now available and one of them will have an orange outline (it's the one that Tableau advises you to choose). To use an option, click on it.
Let's do a quick example using Sample - Superstore:
- Simultaneously select Order ...
