First, you need to pronounce the Shahada. It is known as the testimony of faith, the first and most important of the five pillars of Muslim. Then, you are expected to purify yourself off any sins and enter into sincere repentance. Visit http://www.wikihow.com/Become-a-Muslim for more information.
Say with all your heart ASHAHADO AN LA ILAHA ILLALLAHO WA ASH HADO ANNA MUHAMMADUR RASOOLALLAH. I witnesseth that there is no GOD but ALLAH and I witnesseth that MUHAMMAD(pbuh) is the messenger of ALLAH. Yes if you say this you are now a muslim, but lip service is not sufficient you have to accept it by your heart and you will have to show it by your actions and deeds. CONGRATULATIONS TO YOU MAY ALLAH GIVE YOU STRENGTH AND SHOW US THE RIGHTEOUS PATH. AAAMMMMEEEENNNNNN.