Zina ( fornication) is one of the most dangerous major sins mentioned in Quran and Hadeeths .
Repenting it needs:
1. Real feeling of sadness and sorrow for committing this major sin.
2. Establishing firm decision not to commit it again under any circumstance.
3. repeating Estighfaar ( seeking forgiveness) day and night especially when you are alone and during Sojood ( prostration).