Sincerity towards Allah is the best type of sincerity. The Muslim is being sincere towards his Lord if he achieves sincerity in three aspects: faith and proper belief, acts of obedience, and morals and manners. Faith does not mean mere wishful thinking. The one who is sincere in his faith is the one who attains faith in the way his Lord wants from him, which includes sincerity in faith, sincerity in intention, and sincerity in fear of Allah, may He be exalted. Not every act of obedience is sincere unless it is done, both outwardly and inwardly, in a manner which Allah likes.
Allah has described the sincere in one verse, in which He says (interpretation of the meaning):
“It is not Al-Birr (piety, righteousness, and each and every act of obedience to Allah) that you turn your faces towards east and (or) west (in prayers); but Al-Birr is (the quality of) the one who believes in Allah, the Last Day, the Angels, the Book, the Prophets and gives his wealth, in spite of love for it, to the kinsfolk, to the orphans, and to Al-Masaakeen (the poor), and to the wayfarer, and to those who ask, and to set slaves free, performs As-Salaah (Iqaamat-as-Salaah), and gives the Zakât, and who fulfil their covenant when they make it, and who are patient in extreme poverty and ailment (disease) and at the time of fighting (during the battles).” Then Allah says, after listing all these attributes: “Such are the people of the truth and they are Al‑Muttaqoon (the pious)”
Ibn Katheer (may Allah have mercy on him) said:
This verse includes a number of great meanings, important principles and sound beliefs.
Tafseer Ibn Katheer, 1/485.
Shaykh ‘Abd al-Rahmaan al-Sa’di (may Allah have mercy on him) said: “Such” means those who have the characteristics mentioned, such as good beliefs and actions which are the effect, evidence and light of faith; the morals and manners which lend dignity to man and are the essence of humanity. These are the ones who are “the people of the truth”, whose faith is sincere, because their actions confirm their faith. “And they are Al‑Muttaqoon (the pious)”, because they refrained from what is forbidden and did what is enjoined, because these things include all good attributes by implication, and because fulfilling one’s covenant is the essence of the entire religion, and because the acts of worship mentioned in this verse are the greatest acts of worship and the one who does them will undoubtedly do other acts of worship. They are the righteous, the sincere people of truth, the pious.
Tafseer al-Sa’di, p. 83
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) said:
Among the things we learn from this verse is that what is mentioned is true sincerity towards Allah and towards His creation, because Allah says: “Such are the people of the truth”; they are sincere towards Allah, as on the basis of these good beliefs they establish faith and belief in Allah, the Last Day, the Angels, the Book and the Prophets, and they establish prayer, pay zakaah, and give what is dear to them to the charitable causes mentioned. As for their sincerity towards Allah's creation, that is included in the words of Allah: “who fulfil their covenant when they make it”, which is one of the signs of sincerity. Hence Allah says: “Such are the people of the truth”, as they are sincere in their beliefs, in their interactions with Allah and with other people.
Tafseer Soorat al-Baqarah, 2/293, 294
You should note that sincerity with Allah is not something that we can make a show of and convince ourselves of whilst abandoning righteous deeds. Rather sincerity towards Allah is in the intention and in the action when we do it and when the means of doing it become available. The one who is sincere towards his Lord will, by means of the sincerity of his intention, attain that (reward) which the one who does the action attains, if something keeps him from doing the action. It was narrated from Sahl ibn Hunayf (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Whoever sincerely asks Allah for martyrdom, Allah will cause him to attain the status of the martyrs, even if he dies in his bed.” Narrated by Muslim, 1909.
Ibn al-Qayyim (may Allah have mercy on him) said:
A person can have nothing more beneficial than sincerity towards his Lord in all his affairs, along with sincerity of resolve, so he should be sincere towards Him in his resolve and in his actions. Allah says (interpretation of the meaning): “And when the matter (preparation for Jihaad) is resolved on, then if they had been true to Allah, it would have been better for them” [Muhammad 47:21]. So his happiness lies in sincerity of resolve and sincerity of action. Sincerity of resolve means certain desire to do an action and not hesitating to do it. If his resolve is sincere, all that is left is sincerity of action, which is doing one’s best and striving one’s utmost when doing it , and not letting anything, outward or inward, hold him back from it. Resolve of purpose prevents him from becoming weak-willed and sincerity of action prevents him from becoming lazy or lethargic. If a person is sincere towards Allah in all his affairs, Allah will give him more than He gives to others, and this sincerity results from true love of Allah and true trust in Him. So the most sincere of people is the one who is loves Allah most and puts most trust in Him.
Al-Fawaa’id, p. 186, 187
A Muslim may be confused as where to begin, and how he should order his affairs by night and day. But this should not form an obstacle for the one who seeks to do good or seek knowledge or call people to Allah. The Muslim may rid himself of this confusion by organising his time. He can allocate the first part of his day for memorizing Qur'aan, and he can organise the rest of his days for seeking knowledge by means of tapes, reading books, and attending classes and study circles, and fulfilling whatever duties he or she may have as a husband or wife, whether he or she is working or not.
What we advise to you is to start and not delay, and to be sincere in your intention so that Allah will make your affairs easy for you.
We ask Allah to help you to obey Him and to guide you and give you strength.
And Allah knows best.
(Source - Islam Q&A)