1- Which type of language it used in this board (C, C++, ... ) ?
C++
2- When i buy the kit from the company what they offer with the board ?
The board comes fully assembled. Some vendors include a USB cable which is the only hardware you need to program the Arduino.
3- Regarding the libraries...are they free of charge ?
I have never found any that required payment. All the Arduino libraries I have ever used were free.
4- Is there any useful manuals and that can help me to get more familiar with arduino ?
http://arduino.cc/en/Tutorial/HomePage
5- if i want to connect the atmega328 UNO with GSM/GPS modems, which item would be the best suited for it GSM/GPRS/GPS-Shield or SIM900.
If you don't have experience with electronics then a shield is usually the best way to go. It makes all the connections between the device and your Arduino.