To answer this, you need two pieces of information, your required voltage and required power (either in Amps or Watts). First, find the DC voltage required. Some common voltages are 12Vdc and 24Vdc. It is important to select the correct voltage, and check it requires DC not AC power.
Next, find how much power is required. This is measured in amps or watts (with watts being the total power, calculated as Voltage x Amps), and you need to use a power supply with more power than your required load. So, if you have a 300W load the very minimum requirement is a 300W power supply. However, you can use a power supply with higher output power, for example a 350W power supply.