The Arabic word for repentance is Taubah which translated literally means 'to return'. Repentance means to leave what Allaah has prohibited and to return to what he has commanded.
Allaah has encouraged the believers to repent with sincere repentance.
O you who believe! Turn to Allah with sincere repentance!
[Surah At-Tahrim Ayah: 8]
In another ayah Allaah has said:
But as for him who repented (from polytheism and sins, etc.), believed (in the Oneness of Allah, and in His Messenger Muhammad saws), and did righteous deeds (in the life of this world), then he will be among those who are successful.
[Surah Al-Qasas Ayah:67]
So we should repent to Allaah and submit before him. Indeed even the Prophet (saws) used to ask Allaah for his forgiveness even though he was the Prophet and was sinless. We see that despite this he used to repent to Allaah more than 70 times a day.
"O people, make tawbah to Allah. By Allah, I make tawbah to Him more than seventy times each day."
Even though the Prophet (saws) was never displeasing to Allaah he asked Allah for His forgiveness, how much more so should the sinners?
The Prophet (saws) encouraged everyone to repent to Allaah as a means of purification from the sins. The Prophet (saws) used to say every morning he used to ask Allaah for forgiveness at least 100 times.
Abdullah ibn `Umar said: We counted in a single sitting the Messenger of Allah, sallallahu alaihi wa sallam, saying one hundred times:
"Rabbi-ghfir-lee wa tub `alayya innaka anta-t-tawwabu-l-ghafoor."
(My Lord, forgive me and accept my repentance, verily you are Acceptor of Repentance, Oft-Forgiving.)
[Ahmad, Abu Dawood, Ibn Maajah]
And the best time for repentance is in the morning and the night, in that we may start our day...