First of all, if some sins happens, you have to conceal it by all means possible, never publicize it. In islam, it is forbidden to publicize our sins (telling you just in case you do not know i already)
Regarding the Forgiveness, the best way to ask for forgivness is Pray tahajjud (last 3rd portion of night) and cry infront of Allah.
You can recite this:
رَبَّنََآ إنَّنَآ ءَاَمنَّا فَاغْفِرْ لَنَا ذُنُوبَنَا وَ قِنَا
عَذَابَ النَّارِ
Rabbana innanā amanna, faghfir lana, dhunoobana wa-qinna
'adhāāban-naar (Our Lord! we have indeed believed: forgive us, then,
our sins, and save us from the agony of the Fire) [surah Ali' Imran;
3:16]
Another hadith states that ask for forgiveness in this way:
اللَّهُمَّ أَنْتَ رَبِّى ، لاَ إِلَهَ إِلاَّ أَنْتَ ، خَلَقْتَنِى
وَأَنَا عَبْدُكَ ، وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ ،
أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ ، أَبُوءُ لَكَ بِنِعْمَتِكَ
عَلَىَّ وَأَبُوءُ بِذَنْبِى ، فَاغْفِرْ لِى ، فَإِنَّهُ لاَ يَغْفِرُ
الذُّنُوبَ إِلاَّ أَنْتَ
The best time that duas are never rejected are
- at time of rain
- during travel
- between iqamat and jamat standing for prayer
So ask Allah for forgiveness specially in those times too.
Best is to do supplications of morning and evening, here is link to them:
http://ittibaa.com/uploads/2/8/2/1/2821629/morning_and_evening_supplication.pdf
These supplications a best way to ask for forgiveness of Allah. so make them a habit (both of you) do the morning supplications after Fajr prayer, and do the evening, after asr prayer
May Allah forgive all of our sins