To become a Muslim you must first take your Shahaadah. Shahaadah is an arabic word which means "witnessing/to testify". Muslims believe everyone is born Muslim until they worship a false god or associate partners with Allah (by giving traits of God to a person or thing that is not God). If you have not continued as a Muslim all your life, and have failed in practising the principle of the Oneness of God, it is necessary for you to reaffirm your faith by testifying/witnessing to the Oneness of God. This is where it becomes necessary to take one's Shahaadah in Islaam.
When you take your Shahaadah you testify belief in One true God [Allah] that has no partners/equals, and in Allah's angels, books, and Prophets, and that Muhammed is the last Messenger of Allah. Before you do this, you should make sure that you want to be a Muslim: Read the Qu'ran, and make sure you understand the core values of the religion. You should become a Muslim with conviction and knowledge for it should be your religion for the rest of your life---there's greater punishment in willingly leaving Islam than there is for failing to be perfect in all of its principles. As a Muslim, you must obey all the laws of Allah, such as learning daily prayers and giving chairty, 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. As soon as you believe in Allah S.W.T as being the One God worthy of worship, and that the Qu'ran is the final message of Allah, and the Prophet Muhammed was sent to explain the Qu'ran with guidance from his Creator [and feel secure in the knowledge that you will always believe this], then you may take your Shahaadah. You don't have to be perfect in the practice of the faith to make your Shahaadah but by taking your Shahaadah you are making the intention to aspire to be as the faithful described in the Qu'ran. You should simply have an understanding and belief in it. For instance, something I personally am not proud of, but when I first took my Shahaadah, I had not kicked my drinking habit, but I knew it was something a Muslim shouldn't do. The most important thing to me though, was becoming a Muslim first, and then I would work on being a better Muslim.
Once you're all set, and ready to make a commitment you may say these simple words (it is suggested that you have two Muslim witnesses to honor your protection in Islam but this is not necessary only recommended):
"Ashadu an laa ilaaha illa Allah, wa ashadu anna Muhammadar rasoolullah."
This translates to mean:
"I testify that there is no god but God, and Muhammad is the messenger of God."
You can say it in English although 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 is recommended.
Once you have taken your Shahaadah, congratulations! You have upheld the first pillar of Islam.
Maashaa'Allah, you are now a Muslimah, a woman who submits to God, and are now part of the Ummah (the faithful to Allah S.W.T). Your country is now the faith of Islam, and your race/family is Allah S.W.T's Ummah, along with the 1.5 billion others in this world.
After taking your Shahaadah it is recommended that you tak a bath or a shower, as was the practice of the first Muslims, so that you can pray to God with a clean body as well as a clean soul. A person who takes Shahadah in total honesty will have all his/her sins up to that point forgiven (but 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.
It should be noted that one cannot be forced to convert to Islam/take the Shahaadah because the Qu'ran states
"Let there be no compulsion in religion..." (2:256)