Bismillah.
Narrated Abu Huraira (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) said, "Whoever established prayers on the night of Qadr out of sincere faith and hoping for a reward from Allah, then all his previous sins will be forgiven; and whoever fasts in the month of Ramadan out of sincere faith, and hoping for a reward from Allah, then all his previous sins will be forgiven."
We say sinning is easy, but atoning for our sins to have them forgiven isn't. While this statement can be debated upon, here's a simple method to erase all your sins: fast in Ramadan, and seek Laylat-ul-Qadr by establishing salah in the last few odd nights of Ramadan. If you do these two things with sincerity and expect reward ONLY from Allah, you have a promise from Allah that you'll be forgiven inshaAllah.
May Allah erase all our past sins this Ramadan.
