Backbiting, backstabbing, or tale-bearing is to slander someone in their absence — to bite them behind their back.
The first step of Isteghfar is to resolve never to commit the sin again. Then you have to, in the case of backbiting, seek out those you may have talked ill of and ask for forgiveness. Thereafter, one has to seek forgiveness from Allah. Should all these steps be taken in all earnest and sincerity, then Allah is the Most Merciful and All forgiving.