If you want to earn some money in Mafia II: Definitive Edition, or just want some free stuff, robbing stores is an option.
There are three types of stores you can rob in Mafia II: Definitive Edition. Gas stations don’t have security, but they also carry the least amount of cash. They’re the low risk, low paying option. Gun shops, on the other hand, carry much more money, but the owner is also likely to try and shoot you. You can, however, also steal guns and ammo. And then there are clothes shops, which have the highest amount of money but also generally have security. You’re advised to grab their cash and get out quick, but you can steal clothes from them as well if you wish.
To actually rob a store in Mafia II, you simply need to go into it and point your gun at the cashier. Some cashiers can even be intimidated by simply shoving or punching them. Pointing a gun in their face is your best bet though. They’ll then give you access to the cash register, so you walk up to it and take the cash it contains. When it comes to robbing gun shops, however, it’s best to shoot first and ask questions later, otherwise you’re likely to find yourself dead. Just remember, you can only rob each store once per chapter, which puts a limit on your ability to earn cash.