This mission will be hard if it's your first time.
Anyway, go to the parking garage in Vice City (Closer to Kent Rosenberg's office) Then, as you drive up the ramp, on the first floor, on your right you will see a gun. Take it, and you will meet Lance Vance during a cutscene. Get in his car he will enter as a passenger. Go to the ally located on your map. Get out of your car, and walk up the right staircase - Lance will take the balcony on the other side. Switch to the assault rifle, and use first person view (if desired). Although crouching appears as a tutorial message, you probably won't want to do that as you can't walk while crouching.
Some friendly people arrive to make a deal, but the fighting starts when a red car comes. Shoot the red car and the other cars that keep on coming. When the first wave of enemies is killed, walk down the stairs (unless playing the iOS version, in which case you're teleported back to the observation point). After a cutscene where two gangsters dirtbikes steal the cash and where one gets shot, take the fallen dirt bike and pursue. Do a drive by and kill him with the SMG - if done quickly enough, you can even kill him while he's in the alley.
Get off your bike, and collect the suitcase that he dropped. Return to Ricardo Diaz, and watch the scene to complete the mission.
After this mission, you will receive a phone call from Ricardo Diaz to unlock The Chase. You will also receive $1000.