How do I repent to Allah after watching pornographic websites? And is it permissible for me to pray after committing this grave sin?

Sincere repentance is obligatory, by abandoning the act, regretting what has passed, and resolving not to return to it. Allah is Oft-Forgiving, Most Merciful, He forgives sins no matter how great they are. It is advised to avoid going online alone, but if it is necessary, then let it be with good company to browse beneficial websites.