eng_osama:
1- Which is better for my case, Arduino UNO or Arduino Leonardo ?
Compare and contrast the two on this website. The biggest difference that I've seen people use the Leonardo for is the ability to emulate an HID device. If the board isn't going to be plugged into the computer, I don't see much need for this.
2- When testing my work on breadboard I connect the Arduino to my laptob using USB cable that transmits both power and signals, but when the board is implanted inside my project, how I connect power to it ? .. or Do I need to connect power or I just need to connect a 5V source to the 5V pin ? Well, does the board have to be wholly implanted or I can remove the ATmega chip and use it in a PCB ? .. if I can, what is power needed then ?
You either need to connect 7-12v to the Vin, which will regulate it down to 5, or connect your own regulated source to the 5V pin, like from a wall wart or battery. It can be less than 5V, but the clock speed will suffer if you go small enough. Read the datasheet for the microcontroller on each board for the exact numbers.
You can also remove the microcontroller from the board and use it standalone (it would be pretty difficult to do with the Leonardo, however). Google Arduino standalone for more information on doing that.