As-Salaam alaikum,
I wish to expand, a little further, on Tawheed in summary.. and observe that from whatever way we look at it, the 4 well-known phrases in Islam encompass Tawheed. These are:--
Subhaan Allah, Alhamdulillaah, Laa ilaha illah, Allaahu akbar and (La hawla walaa quwata illa billah).
Taken together, these 4 Noble Phrases encompass Tawheed (worshipping Allah alone) and qadr (pre-decree).
Ibn Abi Awfaa said, a man came to the Prophet, alaihi Salaat Was-salaam, and said: "I cannot learn the Qur'an, so teach me something that will suffice me. He said, [recite] Subhanallah, Walhamdulillah, Wa La-ilaha illallah, Wallahu Akbar, Wala hawla walaa Quwata illa Billah..." (Abu Dawud 832, Nisa'ee 2/143).
Abu Hurairah, may Allah be pleased with him, reported: "Shall I inform you of a phrase which is under the throne and from amongst the treasures of Paradise? Say, 'La hawla walaa quwata illa Billah'', Allah will say, 'My Servant has submitted himself in intention, action and worship (aslama was-taslama).'' (Hakim, Darabi, Fath al Baari 11/501)
A servant does not control any of his affairs; ho does not have the means to repel harm or the ability to attain goodness except by Allah's Will. A servant cannot change disobedience to obedience, sickness to health, weakness to strength, deficiency to perfection except by Allah's Help.
What Allah wills happens and what He does not will, won't happen... the affairs of creation are tied to His Decree and Pre-destination.... Allahu Tabaraka Wa Ta'ala...