With one button, turn on a red led and turn off a green led

Hello,
I am a beginner in Arduino programming, and I am looking for (even if I am convinced that the solution is easy!) How to make the green button, I can turn on the green LED and at the same time turn off the red LED?
Thank you very much for your answer, I love the programmer and I want to learn more!
PS: I'm sorry for my english. I'ts not my native language !
Asuka.