Repentance in Islam means saying sorry for something wrong that you have done and trying really hard not to do it again.
First, you have to realize that what you did was wrong and feel sorry for it in your heart. Then, you have to make a promise to Allah (God) that you won't do it again.
Next, you have to ask for Allah's forgiveness by saying a special prayer called "astaghfirullah" which means "I seek forgiveness from Allah". You can say this prayer as many times as you need to until you feel that Allah has forgiven you.
After asking for forgiveness, it's important to try to make things right. If you did something wrong to someone else, you should apologize and try to fix what you did.
Finally, it's important to keep your promise to Allah and try your best not to do the same wrong thing again in the future. This is because Allah loves when you try to be a good person and make things right.