He committed a sin and does not know what he should do now
Question

                A person who has commited sin[who knew he is commiting sin -at the time]....now that same person feels such guilt that cant be expressed. What should this person do?.
Praise be to Allah.The Muslim does not have to tell anyone of 
    his sin, rather it is not recommended that he should do that. He should conceal his sin and not expose it. The Muslim should feel disturbed about 
    his sin so that he will be motivated to repent sincerely. It is not sufficient for the Muslim to recognize that he has committed a sin, rather he 
    should remember that Allaah is always watching, so that he will not do it at all. If the Shaytaan makes him fall into one of his traps, he must 
    regret committing this sin and he has to give it up and resolve not to return to it. 
In this way the Muslim will have done what 
    Allaah requires of him, namely sincere repentance. The Muslim must not despair of the mercy of Allaah or think that He is unlikely to forgive his 
    sin or guide him to repentance. For the Shaytaan is eager to make the Muslim fall into sin and then he is eager to close the doors of repentance 
    in his face. 
After he repents sincerely, the Muslim 
    becomes better than he was before he committed this sin. Sincere repentance may alert him to things that he was careless about, so he gives up bad 
    companions who help him to commit sin, and he stops travelling to immoral countries where he committed sin, and he stops watching and listening to 
    things that Allaah has forbidden, namely singing, musical instruments and movies. 
Perhaps he has been careless about these 
    matters but the sincere repentance that Allaah has enjoined awakens him to that. 
The Muslim may forget that Allaah is always 
    watching before he commits the sin, because if he was aware of that he would not have done it. But if he had forgotten, then he should try to 
    remember whilst he is committing the sin that Allaah can see him, and that the noble angels can see him, and that Allaah may decree that he should 
    die whilst he is committing this sin – so how would he meet his Lord in the Hereafter? 
If the Shaytaan manages to ensnare the Muslim 
    before and during the sin, then let him remember afterwards that Allaah stretches forth His hand at night to accept the repentance of those who 
    sinned during the day, and He stretches forth His hand during the day to accept the repentance of those who sinned at night, and that Allaah 
    rejoices over the repentance of His slave even though He is the one Who guided him to repent and made it easy for him, and He has no need of the 
    repentance of this person. 
The Muslim must fear Allaah his Lord and 
    regret what he has done; he must also resolve not to return to it and to do a lot of acts of worship. He must change his environment and friends, 
    looking for something better. It is not permissible for him to mention his sin to anyone, and he should make his repentance sincerely for the sake 
    of Allaah alone. He should also be eager to pray in congregation in the mosque and attend study circles. 
We ask Allaah to help us to do that which He 
    loves and which pleases Him, and to make it easy for us to do good. 
And Allaah is the Guide to the Straight Path.