How can Tawheed be achieved and what is the promised reward?
Question

How can a person achieve Tawheed or true belief in the Oneness of Allaah?.
Praise be to Allah.And blessings and peace be upon the Messenger of Allaah. 
You have asked – may Allaah bless you – about an important matter, which is easy for the one for whom Allaah makes it easy. We ask Allaah to make it easy for us and for our Muslim brothers to attain all that is good. 
It should be noted that achieving Tawheed or true belief in the Oneness of Allaah can only be done by testifying truly that there is no god but Allaah and that Muhammad (peace and blessings be upon him) is the Messenger of Allaah. Achieving this is of two degrees, one that is obligatory and one that is mustahabb. 
The obligatory degree is achieved by means of three things:
1-Giving up shirk (association of others with Allaah) in all its forms, major, minor and concealed.
2-Giving up bid’ah (innovation) in all its forms.
3-Giving up sin in all its forms.
The mustahabb degree is that in which people may vary greatly, and it means not having anything in the heart of attachment to anything or anyone other than Allaah, so the heart is focused entirely on Allaah and pays no attention to anything or anyone else; he speaks only for the sake of Allaah and his deeds and actions are all for Allaah and all his thoughts are focused Allaah, may He be glorified and exalted. Some scholars described this degree as: giving up something that is permissible so as to avoid something that is forbidden; that includes actions of the heart, tongue and physical faculties. 
In order to achieve these two degrees, certain things are essential: 
1 – Knowledge, otherwise how can one attain Tawheed or true belief in the Oneness of Allaah and act upon it if he does not know of it or understand it? Each accountable adult must learn about the Oneness of Allaah that which will make his beliefs, words and deeds correct, then anything more than that is a bonus. 
2 – Firm, certain and deeply-rooted belief in that which was narrated from Allaah, may He be glorified and exalted and His Prophet (peace and blessings of Allaah be upon him) of reports and words.  
3 – Obedience to the commands of Allaah and His Messenger (peace and blessings of Allaah be upon him) by doing that which is enjoined, and avoiding that which is forbidden. 
The more a person achieves these things, the stronger will be his Tawheed and the greater will be his reward. 
Our Messenger (peace and blessings of Allaah be upon him) has explained to us that the one who attains the highest level of Tawheed is the one who is promised that he will be with the seventy thousand who will enter Paradise without being brought to account – we ask Allaah of His bounty. 
In Saheeh al-Bukhaari (5705) and Saheeh Muslim (220) it is narrated that Ibn ‘Abbaas (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The nations were shown to me and I saw a Prophet with a group of men, a Prophet with one or two men, and a Prophet with no one with him. Then a huge crowd was shown to me, and I thought that they were my ummah, but it was said to me, ‘This is Moosa and his people. But look at the horizon.’ I looked, and there was a huge crowd. Then it was said to me: ‘Look at the other horizon,’ and there was (another) huge crowd. It was said to me: ‘This is your ummah, and among them are seventy thousand who will enter Paradise without being called to account or punished.’” 
Then he got up and went into his house, and the people started discussing those who would enter Paradise without being called to account or being punished. Some of them said: Perhaps they are the ones who accompanied the Messenger of Allaah (S). Some said: Perhaps they are those who were born in Islam and did not associate anything with Allaah. And they mentioned several ideas. Then the Messenger of Allaah (S) came out and said: “What are you discussing?” They told him, and he said: “They are the ones who did not perform ruqyah or ask others to do so, and did not believe in omens, and did not use cautery, and they put their trust in their Lord.” ‘Ukkaashah ibn Mihsan stood up and said: Pray to Allaah to make me one of them. He said: “You will be one of them.” Another man stood up and said: Pray to Allaah to make me one of them. He said: “ ‘Ukkaashah has beaten you to it.” 
The phrase “did not perform ruqyah or ask others to do so” means that they did not ask others to perform ruqyah for them. Although asking someone to perform ruqyah is permissible, it is contrary to that which is better. 
The phrase “and did not believe in omens” means that they did not believe in omens based on birds or anything else about which people are superstitious and may give up something that they had decided to do because of these superstitions. Superstition is haraam and is a form of minor shirk. 
The phrase “and did not use cautery” means that they did not cauterize with fire to treat sickness, even if it is proven to be of benefit, because the Prophet (peace and blessings of Allaah be upon him) disliked it, and because no one can punish with fire but the Lord of the fire. 
The common thread in these three characteristics is that they “put their trust in their Lord”, i.e., they attained the highest degree of trust (tawakkul) and they did not pay the slightest attention to the means, and they did not depend on them, rather they depended on their Lord alone, may He be glorified. 
Tawakkul (trust) is the summary of faith, as Sa’eed ibn Habeeb said, indeed it is the ultimate aim as Wahb ibn Munabbih (may Allaah have mercy on him) said. 
In question no. 4203 you will find further discussion of this hadeeth, so please read it. And Allaah knows best and is most Wise. 
Note: Achieving true Tawheed or belief in the Oneness of Allaah cannot be achieved by mere wishing or pretending, or by empty claims that have no real essence, rather it is achieved by means of firm belief that is rooted in the heart, by achieving true ihsaan that is confirmed by a good attitude and righteous deeds. The Muslim must hasten to make the best of every moment of his life and hasten to do good deeds and acts of worship; he should ignore the hardship and enjoy the pain, for that which is with Allaah is precious, for that which is with Allaah is Paradise. 
See: al-Qawl al-Sadeed ‘ala Maqaasid Kitaab al-Tawheed by Shaykh ‘Abd al-Rahmaan al-Sa’di (may Allaah have mercy on him), p. 20-23.