There are a couple of different ways in which a person can convert their AC power to DC power. This article will focus in on how to do this by using different types of power supplies one of which is what is called a switching power supply. Make sure that at all times you follow the directions carefully so that you don't cause injury to yourself.
The first thing you will want to do is figure out what it is that you need the DC voltage to power. How important is the DC input voltage to the circuit or the device that you to need to have powered? Most circuits or devices that you buy will have voltage protection that is built into the circuit. For instance if it requires a 6VDC input then the range that is acceptable could actually be five to eight volts of DC. Make sure to check the specs for the input voltage before you do anything.
Next you need to figure out the maximum load that your circuit will need for it to operate. The power rating that you will find for the AC to DC power supply will need to exceed the actual maximum DC power consumption of it's circuit. You can figure out the total load or current of these components on the circuit by figuring out the maximum load rating for all of the items, like motors, servos, lights, resistors and so on. If you happen to buy a circuit that is already manufactured or an electric device the DC load is going to be in Amps and will be identified on the item already. To figure this out on your own you need to use the Ohm's law which is I=V/R or the current equals the voltage that is divided by the resistance.
There are four different main power supplies for AC to DC. These physical types will include circuit boards, brick like switching power supplies, plugs in the wall and then of course power cords with the AC to DC adapter box. If you are keeping the electronics in one box then use a circuit board or a brick like switching. If you are going for a smaller electronic device then use a wall plug or a power cord.
Next you will need to pick the power supply output. The different types are unfiltered or linear, filtered or linear, as well as regulated or switching. The unfiltered are the least expensive of the output power supplies. The regulated ones are sometimes already built into the designs but the others aren't. The filtered ones are the best because they are designed to be able to actually remove some of the higher frequency noise that can come from a power input.
The switching or regulated supplies are also better because the IC uses a pulse width modulation to do the regulating the output voltage. They usually will have better output than the wall plug or the power cord. This type of power supply will also regulate the output of the voltage under a variety of different loads.
Understand that if the power supply doesn't give your device enough current that the device could actually be damaged. Also remember that it is extremely dangerous to work with electricity so be especially careful when doing so.