Allah s.w.t. has set THE stage for us this Ramadhan:
“Whoever observes fasts during the month of Ramadan out of sincere faith, and hoping to attain Allah’s rewards, then ALL of his past sins will be forgiven.” 
When we fast in Ramadhan out of iman, meaning not fasting because everyone is doing it or to lose weight, but because we firmly believe that this is a pillar of Islam that Allah mandates upon us, and wanting Jannah – then all of our previous sins will be forgiven.
2. Praying qiyam every night
“Whoever prayed at night in it (Ramadhan) out of sincere Faith and hoping for a reward from Allah, then ALL of his previous sins will be forgiven.” 
Scholars said that what is meant by praying qiyaam in Ramadhan is to pray Taraweeh. That’s 11 rakaats minimum (inclusive of 3 rakaats of witr) behind the Imam. If not possible, then do it at home. The idea is not to miss a single prayer.
3. Spending Laylatul-Qadr in prayers
“Whoever establishes the prayers on the night of Qadr out of sincere faith and hoping to attain Allah’s rewards (not to show off) then ALL his past sins will be forgiven.” 
How Merciful is Allah s.w.t. He knows all of our sins and mistakes, those days of low-level of iman, those times when we chose desire over piety – yet despite all of that He loves and wants to forgive us. May we emerge from Ramadhan cleansed from our sins, and our hearts healed.
 Sahih Bukhari – http://sunnah.com/bukhari/2/31
 Sahih Bukhari – http://sunnah.com/bukhari/31/1
 Sahih Bukhari – http://sunnah.com/bukhari/2/28