I have a credit card and at a point I found that I couldn't pay it so for 3 months I paid it with interest (Riba). I have now paid off my debt and, Insha Allah, will make sure I won't fall for that again. Now, will God forgive me? And how do I ask for forgiveness?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Having a credit card and dealing with it, itself is surrounded by many prohibitions.
1. Everyone who demands a credit card has to agree to the terms mentioned in the contract, i.e. that he is ready to pay interest, thus he freely approves of dealing with interest. Therefore, whenever one delays in any payment he pays a certain percentage as interest. Merely signing such a contract is forbidden, since this clearly indicates that he approves of this transaction.
2. Very likely the one who has a such card is practically involved with interest whenever he delays a payment as happened to the questioner's brother.
So, dealing with credit card is permissible only when it is free from the legal prohibitions. The terms of all credit applications should be carefully examined to avoid illegal contracts. Whoever committed a sin should repent toward Allah immediately. For more details about how to make repentance please read the Fatwa: 83145.
Allah knows best.