There is only one deadly sin - unforgivable sin. And that the rejection of Jesus Christ. All other sins - murder, rape, child abuse, lying, hating, cheating and infinitely more - these sins can all be forgiven.
To confess one sins to God is to talk to Him and tell things like, "Dear God - I really blew it today and I am sorry. I lied again to my best friend and I confess to you now that I've sinned against You." or "Dear Heavenly Father - forgive me. I can't seem to stop getting drunk. I confess to you that I am destroying my body and am sinning against You." It really is just that simple.
How does one repent? They turn their back on the sins. Make a conscious and daily effort to forsake sin. And acknowledge that on your own - this is impossible. But tell God in prayer something like- "God, I want to repent of getting high today. Help me to forsake this today. Show me what to do and direct my focus to something else. Help me - before I put my feet on the floor to forsake wickness today." There are no magic words and no litergy to recite. Just talk to God.
Yes, God will know that the sincerity of your heart. He alone knows heart of humanity and the motives for every single thing we do.
Are you going to repeat a sin that you have repented of? Probably. We all do. Confess and repent and start over. The book of Lamentations says that God mercies are NEW every morning
Lamentations 3:19-16 =
"Remember my affliction and my homelessness,
the wormwood and the poison.
I continually remember them
and have become depressed.
Yet I call this to mind,
and therefore I have hope:
Because of the Lordís faithful love
we do not perish,
for His mercies never end.
They are new every morning;
great is Your faithfulness!
I say: The Lord is my portion,
therefore I will put my hope in Him.
The Lord is good to those who wait for Him,
to the person who seeks Him.
It is good to wait quietly
for deliverance from the Lord.