Go to the Pizzeria and talk to Tony about becoming a delivery boy and he'll tell you to come back later. Return the next day to talk to him again.
This time Tony will tell you to get a bicycle or something and to just take the pizzas off the counter to start your delivery. 
To get a bicycle you'll need to purchase one from Consum-R at the Mall for $500.
Once you have the bicycle head back and select the pizzas on the counter to start the mini-game.
The
mini-game has you riding along a path on a 2D plane as houses appear,
some of which will have pizza slices above them. On the bottom
right-hand side of the screen you have three pizzas and you need to
select the pizza that matches the slice above a house. 
The
timing on this is tricky and you need to click on the pizza just as
your bicycle is at the beginning of the house. Playing this game is
harder than gardening but can easily net you much more money per
delivery session.