No map would be complete without a legend. Legends are based off of two things: The layers that are available, and the symbology of each layer.
To access the legend capabilities, open the layer properties of each layer that you want to appear in the legend and click on the legend tab on the left hand side.
After accessing the legend settings, Turn On the legend for that layer. You can give the Legend's Layer a name, Reorder the legend items, and turn off individual symbology for a legend if it is not applicable to your map. After clicking, "Save" on the legend tab, it will appear in the map.
That's all there is to it. You can repeat for any layer that you have created in the map to add multiple layers to the legend.