The last thing you have to do for your project is to choose a power source. Powering your board with a transformer is the most secure approach, since if the lock runs out of power it will not work. With most servos you could still turn the lock with a regular key, so don't worry. But with a transformer, you will need a cable from the lock to a wall socket, which might not be aesthetically pleasing for some. In that case, you can power the lock with a battery but keep in mind that the battery will eventually drain. Any transformer that outputs between 7–12V DC will probably do the trick and the same goes for batteries. The power source you choose needs to have a DC-Jack male connector to fit the Arduino board and do not forget to check the polarity of the DC-Jack. For most batteries, there are battery connectors you can buy; if you have the parts you can solder your own. You can't, however, power the lock via USB since there is not enough power for both the servo and NFC...