Sincere repentance is the most important.

The Holy Prophet (صلى الله عليه وآله وسلم) said,

He who repents for his sin is like one who has no sin.

Kanz al-`Ummal, no. 10174; Mizan ul Hikmah page No. 784

The Holy Prophet (صلى الله عليه وآله وسلم) said,

God's pleasure with a penitent person weighs over that of a thirsty man who arrives at water or a barren woman who begets a child or a loser who finds his lost one. Verily He will make the guardian angels, his limbs and all the points on the earth to forget the sins and faults of the one who repents and turns to God sincerely.

Nahj al Fasahah; Tradition No. 803; Page No. 114&115

Imam Saadiq (عليه السلام) said,

Imam Ja'far Saadiq (عليه السلام) narrates from his predecessors that the Holy Prophet ((صلى الله عليه وآله وسلم).s.) said, "Allah gives abundant sustenance to whomsoever He desires. Similarly the mercy of Allah is spread far and wide for the sinners day and night waiting for the person to repent and be forgiven. The mercy of Allah waits at sunset for the people who committed sins during the day to repent and get themselves forgiven."

Sawab al-A`amal, Page 232

The Holy Prophet (صلى الله عليه وآله وسلم) said,

He who repents is like unto a sinless person, and he who asks God for forgiveness but insists in committing sins is like unto the one who ridicules Him, and he who annoys Muslims is sinful to the extent of palm groves.

Nahj al Fasahah; Tradition No. 881; Page No. 125

See what is repentance according to Masoomeen (عليه السلام)

Imam Ameer-ul-Mu'mineen Ali (عليه السلام)] said

Repentance stands on four pillars: remorse with the heart, asking for forgiveness with the tongue, work with the limbs, and resolve not to repeat [the offense].

Bihar al-Anwar, v. 78, p. 81, no. 74; Mizan ul Hikmah, page No. 787

You can also pray this namaz know as, Namaz of Imam Ali (عليه السلام)

Imam Jafar Sadiq ((عليه السلام).) said that whoso prays the 4 Rakat Namaz of Ameerul Momineen (عليه السلام), comes out of evil whirlpool of sins, as baby comes out from the womb. The Holy Prophet (saws) said that whoso prays this Namaz and recites the Dua'a at the end, is absolved from the accountability of all his sins (provided no more sins are committed). Pray this 4 rakat Namaz exactly like any 4 rakat obligatory Namaz (Zuhr, Asr or Ishaa) on Friday or during the night of Friday, with the following adjustments:

(i) In each Rakat, after recitation of Suratul Faatihah, recite suratul Ikhlaas 50 times.

(ii) After Salaam, recite the following Tasbih of Ameerul Momineen ((عليه السلام).):

Allahumma salli alaa Muhammadinw wa aali Muhammad Bismillahir Rahmaanir Raheem Subhaana mal laa tabeedu Ma-a'alimuhoo Subh'aana mal la Tanqus'u Khazaaa-inuhoo, Subhaana mal laa iz “mih'laala li-fakhrihee Subhaana mal laa yanfadu maa indahoo Subhaana mal laa inqitaa-a' Limuddatihee Subhaana mal laa Yushaariku Ah'dan fee Amrihee Subhaana mal laa ilaaha Ghayruhoo

Thereafter, say the following supplicatory prayer.

Yaa man a'faaa anis sayyi aati wa lam yujaazi bihar Ham abdaka yaa allahu nafsee nafsee Anaa abduka Yaa sayyidaahu Anaa Abduka Bayna yadayka Ayaaa Rabbaahu Ilaahee bi-kaynoo natika Yaa amalaahu Yaa Rahmaanaahu Yaa Ghiyaathaahu

A'bduka A'bduka laa heelata lahoo. YAA MUNTAHA RAGHBATAHU. Yaa mujriyad dami fee u'rooqee Yaa sayyidaahu Yaa Maalikaahu Ayaa huwa, Ayaa huwa Yaa Rabbaahu Abduka Abduka LAA HEELATALI Wa laa ghinaa bee an nafsee Wa laa astateeu lahaa zarran. Wa laa naf-an wa laa ajidu man usaani-uhoo Taqattaat Asbaabul khadaa-I Annee wa azmahalla kullu Maznoonin annee afradanid dahru ilayka Faqumtu Bayna Yadayka Haad'al Maqaama. Yaa ilaahee Bi-ilmika kaana Haad'aa kulluhoo Fakayfa anta saani un bee Wa layta shia'-ree Kayfa taqoolu Lidu-a'aa-ee. ATAQULO NA-'AM Amtaqoolu laa Fa-in Qulta laa Fayaa waylee Yaa waylee, Yaa waylee Yaa A'wlee, Yaa A'wlee Yaa A'wlee Yaa Shiqwatee, Yaa Shiqwatee, Yaa Shiqwatee, Yaa d'ullee Yaa d'ullee, Yaa d'ullee Ilaa man wa mimman Aw inda man aw Kayfa aw maa d'aa Aw ila ayyi shay-in Aljaa-u wa man Arjoo wa man Yajoodu a'layya Bi-faz lihee heena Tar fuz unee Yaa waasial Maghfirati Wa in qulta Na-a'm kamaa Huwaz zannu bika War rajaaa-u laka Fat'oobaa lee anas Sa-e'edu wa anal Mas-o'odu fat'oobaa lee Wa anal marhoomU Yaa mutarahhimU Yaa mutara-ifu Yaa muta-attifu Yaa mutajbirru Yaa mutamalliku Yaa muqsitu Laa amala lee ablughu bihee Najaaha haajatee aS-a-luka bismikal ladee ja-altahoo fee maknooni ghaybika was taqarra indaka falaa yakhruju minka ilaa shay in siwaaka as-a-luka bihee wa bika wa bihee fa-innahoo ajallu wa ashrafu Asmaa-ika laashay-a lee ghayru haadaa wa laa ahada aa wadu alayya minka Yaa kaynoonu,Yaa mukawwinu Yah man arrafanee nafsahoo Yaa man amaranee bitaa-atihee. Yaa man nahaanee an maa siyatee. Wa yaa mad uwwu. Yaa masoolu Yaa matlooban ilayhi Rafaz tuwa- Siyyatakal latee aw saytanee. Wa lam utia-ka. Wa law ata tuka. Feemaa amartanee lakafaytanee maa Qumtu ilayka feehi wa anaa maa maa siyatee laka raajin falaa tahul baynee wa bayna maa rajAVtu. Yaa mutarah imman lee a-idnee min bayni yadayya wa min khalfee wa min fawqee wa min tahtee wa min kulli jihaatil ihaatati bee Allahumma biMuhammadin sayyidee wa bi-alliyyin waliyyee wa bil-aa-imatir Raashideena Alayhimus Salaamuj- alalaynaa

Salawaatika wa raafataka wa Rahmataka, Wa awsia alaynaa min rizqika Waqzi annaD DAYNA wa jamee-a Hawaaaijinaa, aa Allahu Yaa Allahu Yaa Allahu Innaka alaa kulli shayin qadeer.