In your Line, Bar and Area add a name attribute.
Example:
<Line name="# Apples" type="monotone" dataKey="series1" stroke="#FF0000" />
<Line name="# Bananas" type="monotone" dataKey="series2" stroke="#FFFF00" />
<Line name="# Grapes" type="monotone" dataKey="series3" stroke="#FF00FF" />
The legend will pick this up automatically:
http://recharts.org/en-US/api/Legend
By default, the content of legend is generated by the name of Line,
Bar, Area, etc. When no name has been setted, dataKey will be used to
generate legend content alternatively.