Pray to God.. And start reading about Him.. it's good that you came here..
I know He exists.. many people do.. It's a certain thing. And we all find out for certain someday. :) There's a good deal of information here.
You've committed sins that can be mortal.. But were you culpable fully or not? Only you can know. Or perhaps only God for now.
The best thing to do after such a thing is to repent those sins, to mean to not do them again and do your best no to do them again, and pray to God that repentance and contrition and determination, and ask for His help.
Then you go to Confession. But a prayer of contrition first. If it's perfect, out of love of God for His own sake and you mean to go to Confession, you'll be forgiven.
He doesn't refuse to bring anyone home who's willing to walk with Him on His way.
Satanism combined with Heavy Metal leaves a lot of stuff to overcome. . temptations that control your life.. but with God all of these can be overcome and you can be free. :) Freedom is a marvelous feeling let me tell you.
Acts of Contrition
Forgive me my sins, O Lord, forgive me my sins; the sins of my youth, the sins of my age, the sins of my soul, the sins of my body, my idle sins, my serious voluntary sins, the sins I know, the sins I do not know; the sins I have concealed so long, and which are now hidden from my memory. I am truly sorry for every sin, mortal and venial, for all the sins of my childhood up to the present hour. I know my sins have wounded Thy Tender Heart, O my Savior. Let me be freed from the bonds of evil through Thy most bitter Passion, O my Redeemer. O my Jesus, forget and forgive what I have been.
. . . My God, who art all good and deserving of all my love. I firmly resolve, with the help of Thy grace, to sin no more and avoid the near occasions of sin.
Precious Blood of Christ wash me free of my bonds, and O Lord send me your saving angel to defend me from all my enemies. I renounce Satan and all sin.. Help me O Lord, to be entirely yours, free from all sins and to live for you alone.
I love you, O Lord, have mercy on me, and protect me with your saving grace.