Attached are the two pieces of software which you will need.
The .pde arduino sketch and the .py Python program.
Upload the sketch to your arduino.
Make a Twitter account and go to the Dev website. Register an application and note the codes given to you.
Run the Python program after having filled in the various fields to communicate with the Twitter API and the Arduino's serial.
Make sure you have the three modules installed (Twitter, Serial and Time). You can get some help about the installation of modules her