You will get it as you progress with the game.
If you want to get it quickly though, you need to search for police vans (hard or very hard lock), police cruisers (medium lock) and ambulances (easy lock). They are large, easy to spot and easy to access as they are usually guarded by a police officer or few dumb zombies.
There are two more options, but they require you to explore. You can find a lot of locked chests on rooftops or hidden in buildings. They are usually locked with medium lock.
Few quests also require you to open lock, but they are rather rare.
You should get this achievement within five hours of gameplay if you don't look for locks specifically.