Simply overlapping the audio events on the timeline automatically creates a crossfade with the first song fading out and the second song fading in. This should be happening for you already if the events are overlapping on the same track.

If you want manual control, place the audio clips on alternating tracks so that the next one starts before the current one ends, but they don't overlap on the timeline. You can add a fade to the ends of the events by moving the mouse pointer to the upper corner of the event, seeing it change to a quarter-circle icon, holding the button down, and dragging into the event. This will create an area as long as you wish with a fade, either fade out at the end or fade in at the beginning.

If you want even more manual control, select each audio track and press V to insert a volume envelope. This will appear as a thin blue line running through the center of the track. You can double-click this line to add "nodes". You can then drag the nodes up or down to adjust the volume from one point to the next. Add as many nodes as you wish.