Allah promises those who enter Islam that "if they cease, what has previously occurred will be forgiven them" (Quran 8:38), and the Prophet ﷺ told 'Amr ibn al-'As that "Islam demolishes what came before it" (Sahih Muslim 121). For someone who was already Muslim but heedless and then turned back sincerely, the same principle of tawbah applies: sincere repentance settles what is between you and Allah, and He may even replace those evil deeds with good ones (Quran 25:70). What repentance to Allah does not cancel on its own are the rights of other people — debts, property taken unjustly, wrongs done to specific individuals — which still have to be returned or put right; and most scholars hold that zakah left unpaid during years one was Muslim remains a debt that must be paid. So do not let a sinful past paralyse you: "do not despair of the mercy of Allah; indeed Allah forgives all sins" (Quran 39:53). Your obligation now is sincerity going forward, not despair over what is behind you.
Q&A · Repentance & Sin
I only started practising recently — will I still be held accountable for all the sins I committed before that?
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.