Allah's Rights upon His creation are the rights that must be kept the most. Allah is the sole Creator and Sustainer of the universe. He is
the Almighty who created everything with absolute wisdom. Allah is the One who initiated every being from nothing. He is the One who protects humans in their mothers' wombs, as infants, as children and as adults. He, alone, sustains all humans and provides them with food and every aspect of life. Allah said, what translated means, "And Allah has brought you out from the wombs of your mothers while you know nothing. And He gave you hearing, sight and hearts that you might give thanks (to Allah)." [16:78]
If Allah refuses sustenance to anyone he will be instantly destroyed. Allah's mercy is what keeps humans and everything else alive.
Allah's control over his slaves is perfect. His bounties are countless. If this is His role in the life of humans, then His rights are what one must keep the most. Allah does not need sustenance from His slaves. He said, what translated means: "We ask not of you a provision. We provide it for you. And the good end is for the Muttaqeen (the ones who fear Allah)." [20:132]
Allah only wants one thing from His slaves. "And I (Allah) created not the Jinn and mankind except they should worship Me (alone). I seek not any provision from them nor do I ask that they should feed Me. Verily, Allah is the All-Provider, Owner of Power, the Most Strong." [51:56-58]
Allah only wants mankind to worship Him alone and ascribe no partners with Him in worship, and truly be His slaves. He wants them to surrender to His will, as they surrender to His control for the means of their lives. It is only fair to worship only the One who holds the existence of everything and everyone is in His Hands. One should thank Allah who, alone, provides for him by worshipping Him alone. Allah said, what translated means: "And whatever of blessings and good things you have, it is from Allah. Then, when harm touches you, unto Him you cry aloud for help." [16:53]
What Allah requires His slaves to do is easy. He does not want hardship to come to those striving to worship Him. He said, what translated means: "And strive hard in Allah's cause as you ought to strive (with sincerity). He has chosen you (to convey Islam), and has not laid upon you in religion any hardship, it is the religion of your father Abraham (Islam). It is He (Allah) who has named you Muslims both before and in this (the Quran), that the Messenger (Mohammad) may be a witness over you and you be witnesses over mankind. So offer prayer perfectly, give Zakat (compulsory charity), and hold fast to Allah, He is your Lord, what an excellent Lord and what an excellent helper!" [22:78]
What Allah wants from us is to worship Him with sincerity, and to perform religious deeds. Five prayers a day bring forgiveness from Him and purity in the heart. Muslims must try to perform prayer in the best form: "So keep your duty to Allah as much as you can." [64:16]
The Prophet said, what translated means, "Stand while praying, if you cannot then while sitting, and if you cannot then while laying on your side." [Al-Bukhari] Allah also wants His slaves to pay a small amount of their money to the poor, the needy, strangers who have no money, the indebted and to others who are eligible to take money from Zakat. Zakat is so minimal that it does not harm the rich, yet it provides tremendous benefits for the poor. Allah also requires fasting in the lunar month of Ramadhan. He said, what translated means: "So whoever of you sights the (crescent on the first night of the) month (of Ramadhan), he must fast that month, and whoever is ill or on a journey, the same number (of days he misses) from other days." [2:185]
Also, Hajj (pilgrimage) to Makkah once in lifetime is obligatory upon all Muslims, if they are able to do so. In general, we have the obligation to abide by all of Allah's orders, and stay away from all He had made forbidden upon us.
Above mentioned are the duties of Allah's slaves towards Him. They are not difficult to perform. The reward outweighs the requirements by far. The reward is "And whoever is removed away from the Fire and admitted to Paradise, he indeed is successful. The life of this world is only the enjoyment of deception." [3:185]