Hello, I'm not sure because I'm also beginner, but I did some research because I have a similar project involving a 16 relays board and an arduino mega:
The relay board must be powered by external 12V power supply
On that relay board are opto isolators, one for each relay, it's basically a small LED, so the Arduino must provide a small current for just lighting those LEDs, about 5mA for each opto isolator. That's all the Arduino have to do, lighting a LED to turn ON a relay 
You want 48 relays: do you have enough pins, as each relay will use one pin? If you don't, there is still a solution which is, using shift registers (search google "74HC595 arduino")
And here you can find useful infos about relays board: http://arduino-info.wikispaces.com/ArduinoPower
Good luck