The First Pillar of
Islam. Also known as the "Declaration of Faith," the name of one of the most commonly uttered phrases today, since it's used several times a day in prayer for Muslims.
Shadah is a special thing, like the Sacrament of Confirmation in Christianity or a Bar Mitzvah in Judaism. It's a special event in which one converts (or reverts*) to being a Muslim, as well as an important spiritual step for your faith.
Converting to the religion of Islam is simple, one must come to accept the truth in the Oneness of God and his holy books and prophets, the Qur'an and Muhammad, respectively.
Shahadah is an arabic word, which means 'witnessing.' The meaning is twofold, you are swearing as a witness of the reality that there is nothing worthy of worship but God and that Muhammad is God's servant and Messenger. Also, once a person makes the conscious decision to accept Islam, they take Shahadah in front of at least 2 witnesses. This is to provide proof that you actually said this and are a Muslim, as well as allows you to receive aid and guidance from your newfound brothers and sisters.
Before you do this, you should make sure that you want to be a Muslim: read the Qur'an, talk with Muslims, and spend time learning its core values. You should become a Muslim with conviction and knowledge for it should be your religion for the rest of your life, Muslims believe there's greater punishment in willingly leaving Islam, not that people on Earth can stop you. As a Muslim, you must obey all its laws, such as learning daily prayers and giving charity, as well as no adultery and no eating of pork, but those seem minor once you truly understand the more important concepts of submission to God.
Once you're all set, and ready to make the lifelong commitment, you gather around some Muslims, typically in a mosque, and say in arabic with honesty:
"Ashadu an la illah ill Allah, wa ashadu anna Muhammadar rasoolullah."
This translates to:
"I testify that there is no god but God, and Muhammad is the messenger of God." You can say it in english, but all the people who converted in the time of Muhammad, peace be upon him, said it in Arabic. You can acknowledge the Oneness of God in english, but saying it in arabic makes it "official."
Once you have taken shahadah, Congratulations! Masha Allah, you are now a Muslim, one who submits to God, and part of the Ummah, along with the 1.5 Billion others in this world. You should take a bath or shower as part of your ablution so that you can pray to God with a clean body as well as soul; a person who takes shahadah in total honesty will have all his sins up to that point forgiven (but all of his prior good deeds will still count), since God is granting you the chance to start over now that you've turned back to Him.
If you so desire, you can give yourself a new Muslim name. It's not required unless your name is of something un-Islamic, like the name of a god (i.e. Zeuss or Diana), but it's believed that there are blessings in having a new Muslim or Arabic name to symbolize your rebirth or reversion to God's path. I have friends who did and some who didn't. Relax, it doesn't have to legally change, but people should call you by your new name, since it's written that God will call you by that name on Judgement Day. It can be an Arabic name, or name of a prophet or sahaba. I picked "Sulayman," which is Arabic for Solomon, the name of a prophet.
The statement of the shahadah is one of the most commonly uttered phrases today, since a Muslim must say it at least once in their prayers, which are at five times during the day. Also, a person should never be pressured to take shahadah, the Qur'an commands people "Let there be no compulsion in religion..."(2:256) If you're marrying a Muslim, you can marry without converting unless you're neither a Christian nor Jewish.
I remember when I took shahada. I was 17 at the time, and I had been visiting the local
mosque many times in the past 2 months. It all started when I asked a friend who happened to be a Muslim to show it to me. He was very happy with me for showing interest, and politely answered all my questions. I met the
imam who leads the prayer similiar to a priest, who gave me a Qur'an.
Over the course of the next 40 days, I read some of the Qur'an as well as comparative religion books, books that analyzed the authenticity of the Bible, articles on the Quran and science, and asked several Muslim students I knew from school. They were all very open and respectful, always happy to help me in my search for the truth about God. I read what other converts wrote, including Cat Stevens.
To make a very long story short, after I confirmed that all my morals and principles matched those of Islam, I was ready. I showered, and drove over to the mosque. I sat down and talked with my friend for over two hours, going over all the tenets of Islam, rationally turning them over in my mind, looking for holes or contradictions. After I made him swear to me that he wasn't lying or hiding anything from me, I told him I was ready to take shahadah
We went into the center of the room, where the imam fetched a few other people to be present. He carefully asked me if I was sure I knew what this entailed, whether I knew that from now on I would follow God's laws, and was ready to keep this for the rest of my life. I said yes. He told me to repeat after him, but the words were too foreign to me, so we did them one at a time. When I was done, everyone cheered. I felt dizzy. I wound up giving everyone there a hug, as people called me "brother" and the whole room joined together in saying "Allahu Akbar." Actually, I nearly jumped out of my skin when they all yelled Allahu Akbar! as loud as they could, since I was perhaps expecting an applause or something.
* "reverts" in the sense that is because the understanding as taught by the prophet Muhammad, peace be upon him, is that all children are born in a state of "Islam" (surrender, submission and obedience to Almighty God) and it is their parents who raise them up as Jews, Christians, Fire Worshippers, etc. So, Muslims consider it more like returning or "reverting" to Islam. The Universe is Muslim.