You can use the 5V pin of the Arduino to power the sensor.
The term "3.3V TTL level RS232" indicates that the serial connection is with digital signals of 3.3V.
The Arduino Uno uses 5V digital signals, so you have to adapt them to 3.3V.
The conversion of RS232 5V to 3.3V is not hard to do, I can't find it right now. Hopefully someone else knows.
The serial port is used to upload a sketch and for the serial monitor.
But if you use SoftwareSerial, you can create a second serial port for the sensor.http://arduino.cc/en/Reference/SoftwareSerial
That way, you can still send messages to the computer, and have full control over the sensor.