The intro of a song sets up the mood of the whole song and leads into the first verse. For this reason, it is important to think about what you play and make sure it creates the feeling you want as well as grabbing the listener’s attention. Some intros are four measures long and end with a turnaround lick, while others may be simply a short, definite statement. Here are some examples.
This one works well as the opening to a slow Blues in a minor key.
Here are two more typical four measure intros.