Hello Bosun,
"When I want to express my sincere gratitude to someone, can I say the following?"
OR
"When I want to express my sincere gratitude about something, can I say the following?"
You could say the following:
"I really appreciate what you have done for me."
"Words cannot express my gratitude to you."
There are many different ways. Could you give us a context? Who are you giving thanks to and for what? That would help us come up with the perfect sentence.