Hello,
I have to order new items for my first project. I've already got an Arduino Mega. I have a device with RS-232. Arduino should communicate with this device.
Which items do I need? I read about a level shifter?
Please help. 
The usual way to level-shift between TTL and RS232 is with an MAX232 chip.
...R
There are many prepackaged adapter boards on ebay - very cheap.