This achievement is self-explanatory. Drive to the navigation marker that indicates where the shopping mall is, and put out the fires around it with the firetruck hose.
What isn't readily apparent is that putting out fires around the city actually gets you paid. For example, when you're on your way from the shopping mall to the other area you need to fight fires, if you stop along the way to put out other fires you seem to get paid $500 for each fire you put out. This makes it a pretty good way to earn money for car and weapon upgrades later! There are dozens of fires around the city that aren't related to the ones you need to put out for the storyline, so just drive around with your hose on full and blast everything (zombies and fires) and rack up lots and lots of dough.