Malc was meant to be meeting DeSean but he hasn't showed, so he wants you to go with him.
Grabe a bike and follow Malc. On the way he'll get a call from DeSean telling him he's been shot. Malc now wants you to come with him to help DeSean.
Once you meet DeSean, he'll tell you he just got grazed, but Johnny says he'll help him get revenge. Now you need to follow DeSean while he tries to find the people who shot him.
They're only a block or so away, so once you arrive, take out the five or six guys and once they're dead, Johnny will tell them he's going to excuse himself.