"La ilaha illallah". (There is no god but Allah).
This phrase is the bedrock of Islam. The belief in the oneness of God is called Tawhid and it is a prerequisite for being a Muslim. Tawhid is to believe that Allah is one and there is none like Him; He has no partner; He neither begets nor is He begotten; He is indivisible in person; He is eternal; He is infinite; He has neither beginning nor end; He is All-Mighty, the All-Knowing, the All-Just, the Cherisher of all worlds, the Patron, the Guide, the Helper, the Merciful, the Compassionate, etc.

"La ilaha illallahu Muhammadur rasulullah". (There is no god but Allah; Muhammad is the messenger of Allah.)
This phrase is the first principle of Islam. There are two parts of this declaration: (1) La ilaha illallah, (Tawhid) and (2)Muhammadur rasulullah. (Risalah). The belief in Tawhid shapes and influences the entire course of our life. Risalah is the channel of communicaton between Allah and mankind.

"Kalimatush Shahadat" (Declaration of Evidence)
"Ashhadu an la ilaha ill Allahu wa ashhadu anna Muhammadan abduhu wa rasuluh." (I bear witness that there is no deity but Allah, and I bear witness that Muhammad is His Servant and Apostle.) OR

"Ashhadu an la ilaha ill Allahu wahdahu la sharika lahu wa ashhadu anna Muhammadan abduhu wa rasuluh." (I bear witness that there is no deity but Allah, Who is without partner, and I bear witness that Muhammad is His Servant and Apostle.)

There are four other forms of Declaration of Faith. They are:

"Kalimatut Tamjeed" (Declaration of the Glory of Allah)
"Subhanallahi walhamdulillahiwa la ilaha illallahu wallahu Akbar. Wa la haula wa la quwwata illa billahil aliyyil Azeem." (Glory be to Allah and praise: there is no deity but Allah; Allah is Most Great; there is no power no might but from Allah the Most High, the Great.)

"Kalimatut Tawhid." (Declaration of Oneness of Allah.)
"La ilaha illallahu wahdahu la sharika lahu, lahul mulku wa lahul hamdu, Yuhyi wa Yumeetu, biyadihil khairu, wa huwa 'ala kulli shai-in Qadeer." (There is none worthy of worship but Allah: He is One and has no partner; His is the Kingdom [of the whole universe] and unto Him is due all Praise; He gives life and He causes death: In His hand is all good, and He has power over all things.

"Kalimatu Raddil-kufr." (Declaration of the Refutation of Disbelief.)
"Allahumma inni aoozu bika min an ushrika bika shai'an wa ana a'lamu wa astaghfiruka lima la a'lamu innaka anta 'Allam ul-ghuyoobi tubtu 'anhu wa tabar-r'atu'an kulli deenin siwa deen il-Islami wa aslamtu wa aqoolu la ilaha ill-Allahu Muhammad ur rasulullah." (O Allah! verily do I seek refuge in Thee from associating any partner with Thee knowingly; I beseech Thy forgiveness for the sins which I am not aware of; verily, Thou art the Best Knower of all secrets. I repent for all the sins and make myself proof against all teachings except the teachings of Islam. I have entered the fold of Islam, and I hereby declare:- There is no deity but Allah and Muhammad is the Apostle of Allah.)